Flintlock long guns are a non-issue, as far as getting them across the US line goes. They come in the mail, and depending on how they are shipped , will require payment of GST/PST and a handling fee.
If you allow the shipper to send a kit via courier company (FedEx/UPS/Purolator, etc) expect to get done up the hoop on the fees. If the items are sent via the US Postal service, it'll cost you a $5 fee plus the taxes as applicable, where the couriers charge schedule starts at around $50 for the same 'service'.
Hillfolk, Sitting Fox, Track of the Wolf (who are really good about shipping to Canada, from firsthand experience) Wholesale sports, Elwood Epps, etc. can all supply you with various LH Flinter stuff, ranging from factory mass produced kits, to customized collections of high grade parts to build your perfect gun from.
The Rifle Shoppe is another place in the US, that may be worth a look.
Cheers
Trev