Plucking alive or dead wouldn't matter much - the feathers should still disappear when the duration ends.
What about spamming baleful polymorph? You could turn all sorts of critters - mice, snails etc... - into chickens (assuming chicken feathers work for arrows). At 15th-level you should be able to spam that quite a bit each day.
Also, at 15th-level you can cast control winds with enough power to cause tornado force winds over a massive area. (By the way, that's the only spell you need to destroy a small army. Forget the feathers!) Would your DM allow you to use that to wipe out every bird within the spell's area of effect and bring their bodies to you? If he won't allow that, just tell him you wiped out his army using the same spell.
wildshape into a bird and provide the feathers yourself? To be honest, I'd do this the traditional way: have the town's kids collect feathers and bring them to you. Shouldn't take them more than 2 or 3 days.
If you can't find birds, consider using leaves or paper. You simply need something thin and slightly flexible to stabilize the arrow's flight; feathers are just the traditional item to use, not mandatory. (Heck, you could leave the feathers off entirely, but that will greatly decrease range - you're basically making the bows into spearchuckers).
The D&D way: Sneak into the small army's camp and steal their arrows.
Have the wizard teleport to a larger town and buy lots of arrows. Or you and the wizard could just wipe out the small army.
A few thoughts:
-First, what size/shape of feather do you need? A single bird likely possesses over one thousand feathers total on it's body, in the same way I possess over one thousand hairs on the top of my head. If the letter of the law is "provide 1000 feathers", hand them 1 sparrow, mission accomplished.
-Since you likely need feathers of a certain size and shape (I'm sure others know better than I about this), I'm assuming you need the longer wing feathers from duck-sized & larger birds. Can you get 50 useful feathers off of a single Chicken or Goose? If so, 200 chickens or geese should do the trick. At 1gp each, you could buy them someplace and deliver to the locals, they can pluck the birds themselves.
-If it is a matter of larger sized feathers, how about a single, much larger sized feathered creature? Would the DM permit there to be a Roc, a Griffon, or some other bigger creature that could provide thousands of feathers? You might be able to ask for them, than see if Healing magic can restore the lost feathers quickly, or otherwise be in the debt of the animal or magical beast.
-You're a 15th level Druid, spam spells. To accomplish this specific task, you could:
Commune With Nature: inquire to the presence of the birds you need, or perhaps their nests already filled with shed feathers.
Call Animal + Speak With Animals + Charm Animal or Dominate Animal: Repeat the process enough times to bring the appropriate birds to you and get them to be willing to have the necessary feathers plucked.
-Finally, YOU ARE A 15th LEVEL DRUID. Offer your services against a small army, you can have so much more effect than one thousand arrows. You're talents are wasted on feather gathering. How about you offer the following:
Before the battle, use Rampart, Animate Plants, Wall of Stone, Stone Shape, Control Water, Obsidian Flow and Move Earth to create whatever sort of massive walled fortress, moated defense, tree-fortified, glassed landscape you feel like. They have you picking feathers, you could be creating an elevated, defensible metropolis in the span of 2 weeks. Once that's created, you offer to Transport Via Plants to where the enemy is located, cast Control Weather before revealing yourself to whip up some impressive environmental effects, Wild Shape into something awe-inspiring like a Huge Air Elemental and calmly explain that you are a high level Druid speaking on behalf of the locals you are defending, politely requesting they back down, throw a few powerful spells around to demonstrate your point or Euphoric Tranquility the enemy commander into declaring the war ended, than Word of Recall yourself back. If the enemy arrives and STILL feels like taking them on, you've got a bazillion possible area damaging spells. One casting of Earthquake should finish an army.
...And you've got a party that I'm assuming is close to your level, any of which could contribute a variety of helpful skills better than gathering feathers.
Probably very risky ... the little intelligence we have so far says that they have over 10,000 and some powerful spellcasters who are likely higher level than us. We're trying to keep our resistance on the downlow for now. Getting more info on them is definitely part of my plan.
The European way: buy several whole dead geese at the market for chump change, or go out geese hunting.
Ah. When you said "small army" I was imagining something in the low hundreds, maybe a bandit group. I'm pretty sure 10,000 backed by high-level casters counts as a large army by most standards. And frankly, I don't see how a few hundred arrows will make a difference in a confrontation on that scale - you might be better served looking for other ways to defend the townsfolk.
This. Fletching arrows typically calls for the flight feathers, and each goose has a whole bunch of them. Feathers should not be a major resource block.
Have the wizard cast fabricate.
Even at minimal caster level you'll have enough for an army. Save your manpower for other tasks.
It's my understanding that the townsfolk have put in requests to other towns for supplies, and come up short on feathers, but I may be wrong on that. Will have to consider that.
I know that fabricate was being suggested, but I think the DM brought up some issued why that would be difficult. Something about her not having enough ranks for the skill check, I think.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.