I haven't been able to locate any other wireless USB adapters with OS X support.
It should rather be able to handle 108 Mbit/s or higher.
Edit: As I'm not a Mac user, I totally mixed up what Airport devices do. Now I realize what an Airport Express device does, thanks to The Devil Tesla, so I edited my question.
I prefer an USB adapter for the portability's sake, even though an access point would do the job.