You should be able to find something decent in Philly. Check out Coops, Craigslist, garage sales, and auctions. Like mentioned before replacing pedals and/or tires shouldn't be a problem. Your biggest problem will be finding bikes in your size.

If your current bike is to small it probably isn't worth putting money into it. However, do ask what is wrong with it. You might be able to find parts through the internet for cheaper, and do the work yourself.