Do a quick search for Tower and the model boat you have. Be sure you are not using the terms rack and tower as the same. There are plenty of folks that will share stories of purchasing and installing for most model boats.
The Rack is what you put on the tower to hold your boards. So no use in buying a rack until you find the tower you want. Although, there was one vendor I saw listed that would sell the whole kit, (tower, mounting hardware, racks and even tower speakers) Probably crap speakers though.
I love having speakers on the tower just for the redneck yachtclub!! I have mine tilted down a bit so we can fade most of the music to those and hear it outside the boat well.
Good luck,