It's easy:
1.) Talk with manufacturer/dealer/person from whom you're buying, you'll likely have to lay down a deposit.
2.) Call Foreign Affairs and get an IIC (International Import Certificate), doesn't cost anything, they need details on the gun you're importing and your PAL. It takes about a week to get it.
http://www.international.gc.ca/controls-controles/about-a_propos/impor/IIC.aspx?view=d
3.) Forward the IIC on to the exporter on the US side, they'll walk you through the remainder of the process which is really simple.
4.) Pay the person from whom you're buying it and they ship it to the exporter.
5.) Exporter takes it from there, takes care of the paperwork on the US side and when it's all done ships it to you.
I just used this guy for an action, started end of Jan and I should have it in the next couple weeks, there were some delays on my part, but I think if everything goes smooth 6-8 weeks and you should be done.
http://www.gadbois.us.com/
Another option is Questar or Prophet River, either one will treat you right also!