Part support boils down to the hobby shop not carrying the parts for a certain product because it isn't a good seller in the area. It has nothing to do with not being able to get them. If a hobby shop is saying that, they are full of it.
HRP, Great Planes and Horizon are the 3 largest distributors by far. They all carry Mugen,HB, AE and Losi (Horizon) Xray is however different and Kyosho is just plain weird to say the least. Xray and Kyosho pretty much require a seperate account and most dealers are not willing to go through the trouble. Shops are alot better off spending more money with just 1 or 2 dealers than mixing it all up. Ofna/Jammin/Hyper is getting a little weird as well. I have noticed that Horizon is picking up on Hyper parts as of lately.
Part support should never be an issue with the basic name brand cars. I work for a shop part-time and RC Driver the rest of the time. When I place an order, if Great Planes does't have a part, I check with Horizon then down to HRP. We stock HB,Losi, and Mugen. No Associated because it simply doesn't do well in our area, However we will carry parts if someone request that we do it.