You need to be more specific than "I can find the D12-5 everywhere". Where is "everywhere"? Is that local Hobby Lobby stores only? Retail stores sometimes only carry limited variety of motors to match the specific kits they sell. If they do not sell a big heavy model that requires the D12-3 and only sell the lighter higher flying, longer coasting models that use the D12-5, they will only stock the D12-5.
Hobby Lobby sucks, sadly everything is online now man. Hobby Lobby stopped carrying the estes 29mm and e motors and even the estes pro series composite motors.