When I was in high school, I learned to play the guitar. Unfortunately, I set it down and haven't played for 15 years. Now with tech catching up, I so wish I had continued. Garage Band, connecting instruments to Macs, and just being an artist is so much fun with technology. I cannot imagine being able to record and work on my own music. I may have just taken a completely different career path... life would be so much more fun as a musician... than working as a consultant/lecturer/writer.
I don't personally have any experience for this, but I did find one for my daughter's keyboard a few months ago by searching on Google + "Mac compatibility." Just find one that states it will work with a Mac. I have found Google to be the place to go to find direct information from cable/music component providers. Sorry I cannot provide more help, but my help is to state don't give up on Google. Just keep modifying your search until you find a product page showing the Mac compatibility and system requirements that shows will work with a USB port and Leopard plus Garage Band (your version). Also, make sure you note that you don't have an audio/mic "in" on the MBA, so you need a device that connects to the MBA via a USB cable.
Best wishes, and keep on jamming!