i do a bit of wardrvifing but mine is a pci card. I helped code one wifi sniffer app in linux. The main reason why not a lot of adaptors are supported is the guys that code need that adaptor to get it working. They are still expensive for freeware developers to afford. You'll notice on a lot of those sites they will ask for hardware donations.
My advice is to check out a lot of apps and see what they supports and buy an adaptor based on that. USB might be a bit tricky to find.