I'd prefer to buy local, but there aren't any stores within 50 miles of my house that sell high performance xc gear. The places I buy from online all have brick and mortar stores: Berkshire Outfitters, High Peaks Cyclery, WebSkis, Caldwell Sport, Boulder Nordic, Gear West.
I've bought a ton of stuff from Sierra Trading Post over the years, always great product, great service, and pretty good prices. Bought some stuff from Backcountry.com as well, seems to be a good resource. I like Ebay and CraigsList as well, you've just got to know what you're shopping for. I'd shop local but there is no 'local' within an hour++ of my home. OGE is pretty good if I feel like a road trip. Keene Valley is just too retail for my tastes.
Small sample but so far backcountry.com and Sierra Trading Post look like reasonable choices. Not sure how many of the votes for BC.com are really for steep and cheap. Actually used BC.com yesterday and the online help/chat was primo.
Right now we're using Liftopia and Backcountry.com.
The Clymb is also part of network we joined. I like the look of their ads.
"If it's my mountain, I'm installing a fixed grip and telling the lifties to stuff the beginners into chairs as aggressively as they have to to keep it going full speed." —Brownski