Probably 90% or more are built by the owners or purchased used. Most of the RTR boats that you'll see are more than likely a Gas boat like the Warehouse Hobbies boats. Not too many of the RTR or RTF boats made don't really fit in a class, I call them "Tweeners". Those would be the over .129 and under .21's and the .32's for instance. I think that the .32 would fit in the "C" class in IMPBA but it's a class that isn't used much.
If you can approach a club with enough boats to fill a class, say Nitro Hammers as an example you may convince them to let you race. I used the NH's as an example because imagine the problems of having 6 Miss Bud's all on the water at once!
The IMPBA does have what is called the ABS class but as far as I know has not been used. The restrictions are a .16 max, pull-start only and mufflers, no tuned pipes and naturally an ABS plastic hull. Not too many boats fit in this category, maybe a Kyosho Nitro Viper and a ProBoat Nitro Fury.