Except for what the exchange has been doing to us lately, the cheapest source for those parts will from online vendors in the US. If you haven't got one, get a US mailbox in Blaine and get items shipped there and go pick them up. Shipping items into Canada is so expensive. Avoid buying at local dealers unless you have cash to burn.
A couple more online sources are Tweetys.com and MakariosRV.com. Be wary of Amazon and ebay for warranties, returns and problems like say a missing part. I wouldn't buy things like furnace, fridge or water heater there.
I doubt bundling will get you the best pricing. If you have a part number and/or item description, spend some time googling to find the lowest shipping + item cost. Can save a ton of money, especially on big ticket items like fridge, furnace and water heater.
For a local parts source, try Ted's RV in Langley on 208th near 40th ave. Very knowledgeable and helpful owner and he can get you some hard to find items. Maybe there is someone closer to you on your side of the river?
If you plan to camp in the US much, now would be a good time to get Nexus cards so you can pick things up quickly at a US mailbox.