Many tires coming from Japan (and even Korea) are having production problems right now (some are effects of the japanese quake months ago).
If you "need" tires, then you "need tires" and 225-50-15 are not an overly common size for performance brands. Maybe you can be our guinea pig
225-50-15, maybe try 225-45-15 as well.
Hankook RS3? Toyo RA1 or R888? come to mind to me, but I don't know if they also have production issues.
Last time I used Yokes they were A032Rs. Loud tire not nearly as sticky as the then current Comp TA R1 or Victoracers
You might look over the popular scca "street tire" brands but most don't come in 15" sizes.