I agree with the "if you want a spell that's not from the phb, then it will require a side-quest" I'll use it on my next campaign. wizards and clerics will have access to the phb spells, and maybe a few low-lvl "extra-phb" spells, if they want more than those, they have to research them, spend time on the library, or...use my custom merchant, he'll sell anything you need, and give you whatever info you need, you just have to pay. don't have cash? it's nothing that a small job won't solve, like retrieving a stolen item, colecting herbs, colecting money from a debt...
If a player is planning on chosing that kind of spells, they'll just have to give me the spell's description. then, if I see that it fits the campaign, they may chose it naturaly, or have to search for it.
In the case of a bard, I'll allow him to know the spell, but only as part of a gossip, or a talk between spellcasters, only after he actually learns how to do it, will he be able to use it. I'm still trying to figure out how to make this work for the druid...