I was searching the Internet for something else the other day and I came across the Hohner Celeste. According to the info on one site: The Celeste harmonica is the latest in the range of Hohner tremolos, it has twenty four double reeds mounted on a plastic comb, comes in a wide range of tuning options, and is tuned in complete octaves.

It's reasonably priced and appears to be available in all 12 keys.