It depends really. For mockup stuff I use music from Featured Audio and sound from http://www.freesound.org
If I’m really happy with the music, I contact the author and see if he’s affordable available.
Just keep in mind its not cheap and it can’t be cheap if you want custom made music and own the full rights.
The way I do sometimes is to use DrPetter's homepage - sfxr because it is damn simple and real small (50kb !!!) .
Problem is that it makes a 8bit or 16bit sound. But with little work on Audacity and no real advanced knowledge of sound engineer you can get a “good” sound and all for free.
The basic tricks would be to add some reverb and delay to make the sound more natural for a start. Then Audaicty has all kind of sound effects and plug ins that allow you to modify your base sound.
It is fine for games like pinball, space shooter, mario and other non-realistic games.
If you need more realistic sounds then you need to go for those real pros.