Welcome to the Music and Sound forum on GD.net. Pull up a chair, listen to some tracks by your peers and discuss all things music and sound. We're happy to have you. Please check out the FAQ link for some basic (and mostly common sense) approaches used in this forum. If you have any questions, feel free to PM me.
A quick search turns up plenty of results -- most sites have samples you can listen to before purchasing, so you should be fine to go with anything you like as long as they have a price that is acceptable to you and appropriate licencing terms (this shouldn't be a problem, but just check before purchasing) for your intended usage.
I've used AudioJungle (third result from my search above) in the past and been pleased with the experience.
Have you considered talking to a composer about the price of having music created specifically for your game? Royalty free tracks from a library can be a great way to cut costs or if you need music to drop right into a project now, but you might be surprised how affordable some composers can be -- if you haven't already done so, it may be worth at least asking for some price estimates. There was recently a discussion of licensed vs original music if you'd like to read some thoughts on the matter.