Well, I try to buy as close to me as possible and avoid shipping altogether. However, there are some species/morphs that you just don't find in Canada.
For instance, I am driving down to Seattle in a few days to pick up a 0.1 CB'03 Olive/Black African House Snake. I'm pretty certain I'll have to find her mate in the US as well.
If it weren't for CITES restrictions, I probably would've bought a Corn Island boa from the same breeder as well. However, I am going to try to talk her into doing the paperwork and get it for me anyhow.
I am definitely going to pick up some ackies up here though, although I have to say I have been tempted to pick up some Blue Spotted Timors from down South instead.
So, in my opinion, there would probably be a lot more buying from the US if it weren't for the paperwork, especially for more exotic species. There is just a lot more variety down there. However, I don't see any point in purchasing species from the US that are readily available here, unless it's to acquire new blood lines.
1.0 Pastel Ball Python, 1.9 Normal Ball Pythons, 0.1 African House Snake, 1.0 Savannah Monitor, 0.0.1 Argentinian Horned Frog