My go to is Missouri Star. They have excellent selection and service with average prices. Their shipping terms to Canada are terrific. Flat $10US for any size order. $5US when they have free shipping in the US. Plus (and this is a big plus) they ship Fedex Express. My last shipment arrived 2 days after it left Missouri. I believe one of the original owners is Canadian and they give shout outs for Canada Day and Canadian Thanksgiving (or Thanksgiving as I like to call it) which is a nice touch.
I have yet to order from Fat Quarter Shop. Same as MSQC as far as price and selection....maybe even a bit better on selection. Last time I checked their terms they offer free shipping to Canada on orders over $200.
Another go-to pre pandemic was Hancocks of Paducah because of their awesome clearance section. In the range of 2500 items mainly by the yard but also precuts, kits, patterns. Sale prices in the $5,6,7$/US range with frequent promotions for an extra 10 or 15% off. They shipped by weight so shipping not great and averaged 3-4 to arrive at destination in SW Ontario. I say pre-pandemic because they stopped shipments to Canada for months. When they opened up I was going to place an order for 5m.....but they have jacked their shipping prices....it was going to cost $60US to ship 5m. Hard pass on that one!
I have tried other outfits when searching for a hard to get fabric and most are the same....good selection and similar pricing but shipping costs are high.
Another bonus but a kick in the pants to your local Cdn shop.....80% of the time I do not get charged HST. I was told that the volume of packages coming across the border is so high that they can only randomly assess for HST.