You're pretty stuck. You're going to have to find someone with a JR radio, or buy a JR buddy box. But last I checked, they were hard to find, and Horizon was backordered on them.

When I was learning to fly, I was in the same boat, just about the only guy with JR gear. I partnered up with another beginner, and we used eachother's TX as a buddy box.

Oh, I think I saw a website somewhere that claimed to have plans for a converter cable or box or something, but it was strictly home-brew, and I've no clue if it would work or not.