I hate to say it, but pectoral muscles (what you're working with push-ups) aren't the same as breast tissue. A good set of pecs will make your breasts look nicer and feel healthier, but it's not going to give you improved cleavage or anything. Take it from someone whose body type makes this abundantly clear
On the cheap and painless side: push-up bras, silicone inserts, the TakeOuts VogueBoy suggests. Also, you can go an awful long way by simply paying attention to your clothing choices...plenty of options to enhance a limited bustline.
On the more expensive/painful side, it would be implants.
As you age (and certainly if you become pregnant) your breasts will grow a bit, and you'll probably eventually be glad to have started from an A (moving to a B or C) than from a C. OTOH, that's the time when all those push-ups really will pay off!
Good luck
And remember, I'm living proof that it could be worse!