I always feel good about patronizing the good Local Bike Shops. They are here for us... those that I deal with are consistently reasonably-priced, particularly with service fees. If you need a part in a pinch, you can often find it the same day at a well-equipped LBS. (I've also got to give a shout-out to Boise Bicycle Project. They are a good source of knowledge, as well as binloads of good used parts that are still 100% functional at amazingly reasonable prices.)
However, as a guy who goes through a considerable number of tires, tubes, and other "consumables" (up to and including brake pads, chains, chainrings, cassettes, etc.), I'm always on the lookout for good prices as well as reliable shipping and delivery.
And - my new favorite parts supplier is not only out of town, it's out of country! Maybe 5000 miles away!
Chain Reaction Cycles is across the pond - in Northern Ireland. They have a fantastic inventory of stuff at very competitive prices. I've ordered from them several times, and their service is really quite remarkable!
Most recently, I placed an order for two tyres (they sell tyres - but they substitute nicely for tires!) and three tubes. I deliberately ordered a little over $60 of merchandise, which qualified me for free shipping. The order was placed on March 16 (Monday). I got an email from the shipper (DHL Express) on Tuesday... enroute, scheduled to arrive Thursday. I got a follow-up email from DHL on Thursday morning, "Your delivery is today." And by 10am, I was slicing through the packing tape on the box. Sah-WEEEEEET! (I've ordered from the two big state-side bike mail order companies - "N" and "P" - for years, and it typically takes a week to 10 days for the stuff to arrive.)
If you occasionally go to an out-of-town supplier, I'd suggest you put Chain Reaction on your "go to" list. Remember: tyres = tires, spanner = wrench, etc.