I think the fastest way to get some useful information would be to contact a US supplement company that already sells in Canada, to discuss how they are currently distributing in Canada.
Then armed with this information, you can then approach the new US supplement company that do not yet have a presence in Canada, to discuss wholesale prices and Min Order Quantities.
Obviously it goes without saying that you need a plan of who you are going to sell to, for example direct to customers, or business to business.
Goodluck.