In the Mt. Shasta area places to buy supplies and rent gear are:
The Fifth Season in downtown Mt. Shasta is the place for anything / everything related to the mountain. They are open everyday and sell or rent most equipment that you might need. They also have a collection of hiking clothes and gadgets as well as posters, books, etc. They even run a sno-phone (530-926-5555) for the latest info on mountain conditions. One of the best overall sources of useful information related to climbing the mountain that I've seen is the "Mt. Shasta Climbers's Review" published by The Fifth Season. This wall poster is printed on both sides and is packed full of photos clearly showing the different routes as well as various charts and text describing conditions, routes, etc. You can either pick one up in person or you can call them and have them mail one to you for a very reasonable price. My experience is that they are a friendly helpful bunch of folks.
House of Ski & Board While mostly a ski & Snowboard shop, as the name implies, they also rent hiking/climbing gear: boots, crampons, ice axe, etc. They even rent climbing helmets - something that you can't get at The Fifth Season. We've rented helmets here and looked around a bit. They have clothes, goggles, books, etc. Lots of good stuff and a friendly staff.
REI is a nation-wide chain of outdoor equipment stores providing "Quality Outdoor Gear and Clothing Since 1938". In addition to the retail stores they also have catalog sales. While anyone may shop at REI members receive an 8%-10% dividend on purchases made over the year. Membership is a one-time $10 fee and you can sign-up at any store. Among other things membership gets you on the mailing list.
REI backs everything they sell with a 100% satisfaction guarantee. In addition to many name brand items and lines REI also has their own line of equipment which is usually a fairly good blend of quality, functionality, and price. Of all the stores in the Bay Area REI has the largest selection of gear and clothing for the greatest number of outdoor recreational sports (camping, hiking, climbing, bicycling, canoeing, etc).
Redwood Trading Post, near the intersection of El Camino Real and Jefferson Avenue in Redwood City, is one of the best kept secrets on the peninsula. The folks who run the store manage to fit a lot of stuff into a small place. The staff is helpful and available. In addition to clothing, tents, backpacks, boots, stoves, etc. they are one of the few places that carry a good amount of do-it-yourself items, such as foam, webbing, buckles, pack cloth and other materials by the yard, etc. Great store with many items that are hard to find elsewhere.
The Down Works is another one of those great little stores that packs a lot of stuff into a small space. It is run by the owners, Nick & Shelley Gravem, who are knowedgable, helpful, and accommodating. They've been in business for 27 years and started out making their own down quilts and bags (as the name implies). Over the past 15 years or so they have been broadening their retail portion of their business and now carry a rich line of backpacks, stoves, sleeping bags, tents, some clothing and boots, etc. They are one of the few retailers in the (extended) Bay Area that actually carry ultra lightweight gear, such as golite and other lightweight product lines.
The North Face has top quality (and top dollar) mountaineering gear and clothing. They have complete lines of clothing that are made to layer and work together for the ultimate in expedition-class gear. If you've got the money, and you want to be able to take on Everest, this is the place to visit.