Depending on how different your breasts are, you might not have to go the full prosthetic route. I have some bras that have removable padding in them, so what you could do is buy a bra that fits your larger breast, take the padding out that side and leave it in on the smaller side.

Also depending on your size, you might have better luck finding bras online than in stores. And you're very right; lots of women struggle with this very issue. My sister is almost 30 and has the same problem. I've been trying to get her to get "really" sized for a bra so then she can actually buy some that fit...