Other than buying/finding them, is there any way to obtain rituals?
I ask because my DM has basically told me flat out that I won't have any opportunity to acquire them any time soon. I found this odd, considering a level 1 ritual costs 50g and (I assume) would be fairly common.
Research. Just tell the DM your character is going to spend a week inventing ritual X for himself. Pay the cost, fluffed as outlay for magic inks and gathering the needed information, and you get the ritual. That would be my answer to all the various "but you can't find it here" kind of arguments. You're actually giving up time as well as the usual cash cost of the ritual, so it isn't like he can argue you're getting something for cheap. Since rituals ARE a standard part of the game your character deserves them. He spent the feat or has the class feature.
I'm definitely not on the same page with you. Players IMC have access to rituals when they seek out npcs that have them in game, or find ritual books as treasure, or find them graven on ancient stone tablets. They do not just get to spend the money and learn the ritual; they have to find the ritual first.
Obviously, everyone plays a different game, and play how you like; but I strongly feel that the finding part of acquiring a ritual is something that is in the dm's hands, not the players, and I would feel like the players were trying to usurp my authority if they just declared that they got such-and-such a ritual.
As an example, in the biggest town in the pcs' starting area, IIRC all rituals of up to 8th level that are in the Players Handbook 1 are available, along with a couple specific ones from other sources and a couple of homebrewed ones. Meanwhile, one of the wizard pcs lives in his master's tower (master has been gone on a journey/missing since before the game started), which contains a number of other rituals- but they aren't easily accessed; they are warded, trapped or hidden in shelf after shelf of books.

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