Below is a list of places I have (or soon will) purchase from:
Hical.ca - promag magazines and maybe an auto bolt release if I'm feelin too lazy to do it myself haha
ca.wholesalesports.com - purchased my sr-22, red dot scope and case for a decent price
dlaskarms.com - purchased a set of 4 full length rails
Questar.ca - purchased a pws T3 mag release
Tactical-mod.ca - (airsoft and paintball website) purchased a Magpul PTS AFG (angled fore grip). I would buy this from hical if you are purchasing other items as well since the fore grip ended up costing about the same after shipping if I bought it from hical in the first place (depending on how much hical charges for shipping on multiple items)
Those are the places I have found in Canada that had parts I wanted. I find that the best strategy is to browse the sponsored stores for the forum (all those advertisements at the top of the page) and decide on what parts you like. I also did a lot of google image searches to see what the parts looked like installed to help me decide. Once you have a list of parts you want, make a list of places that supply them or send an email to see if they can order it for you. If that doesn't work, start a thread about where to purchase the item and a dealer might PM you