I like to get wide backings from Connecting Threads - I've never been able to get free shipping to Canada from Fabric.com, but Connecting Threads ships free at $75. Their quality is good, but the selection is more limited.
Hamel's Fabrics in BC has a good selection of wide backings:
http://store.hamelsfabrics.com/store...&categ_id=2477
I also buy wide backing at my LQS. Even without shipping, a Canadian store price of $20/metre is equivalent to $13.50 US/yard, so the online prices aren't as good as they seem. It depends what's available where you live/shop, though. If your fabric stores are charging $40/m, then online is definitely worthwhile.
Edit: Not that there's anything wrong with fabric.com... I just find that they don't have the best deals once you factor in shipping and exchange.