edit: removed link... Thought I found some free downloads. they wanted $100 a CD but you could use their stuff royalty free. However, I won't advise spending $100 on a mission...
Might I add another possible solution. I am unsure of the age of some of the mission makers, but if you take a night away from OFP and hit a local music scene you might find a band or two willing to allow in writing use of their music. As an ex-band member, struggling artists were always quick to allow anything that might mean exposure.
They may even write something specifically for your mission taking inspiration from your mission idea. Again, this is assuming you are interested or have a local music scene.