R. Nicholls Distributors is the official importer of HK in Canada. I believe they only source from HK Germany and not HK-USA. They are absolutely terrible to deal with if you are not LE/Mil. Most HK shooters in Canada don't even bother to try to contact them.
I have been successful getting HK parts from Wolverine Supplies, a retailer in Manitoba. It can take quite a while for them to get parts if they don't stock it and they either don't stock a lot of HK or it goes fast. If Wolverine has it they probably are familiar with what it takes to import to the US. I have no idea myself.
Out of curiosity, what are you looking for?