Finding Gundam kits at hobby shops is sort of a hit-and-miss thing. Most Hobby Town USA stores can special-order kits (saves you from the shipping cost). Really, you just have to look around. Most hobby shops cater to their locale, and stock whatever people seem to be interested in. The Hobby Town USA where I live used to only stock one or two Gundam kits at a time, until I started buying them every now and then.
If you live anywhere near Denver, I'd recommend Colpar Hobbies. Both of their locations always have a fair amount of Gundam stuff in stock.