I use Rotella T6 for oil now. Canadian Tire price is $42... but at Walmart in Buffalo it is 1/2 that price. Seafoam is much cheaper in the States too - $13 here, $9 there.
General stuff like plugs you can get anywhere.
For air filters....I went to K&N's when my Yamaha dealer was out of stock on OEM but they had a set of K&N. Don't have to replace them, just clean and go.
When I replaced my fuel filter I went OEM from the local dealer. Some people use an automotive filter.
Keep in mind that we get a 15% discount from Pioneer Motorsport in Chaffee, NY (close to BongoBob's). Watch the exchange rate though because it can make the dealer pricing close enough to buy local.