Practically Everything
eBay: You can find almost anything on eBay, especially hardware items and motors. There seem always to be foot throttles; plenty of steering wheels; gearshifts; bow eyes and handles; control cables; propellers; seats; deck plates; instruments; trailers; aluminum for jack plates and steering arms.... Outboard motors sell briskly on eBay, as well as parts. Start searching for stuff as soon as, or before, you begin building your boat. And keep going back. Some of the suppliers listed on this page, like West Marine and Harbor Freight, also have eBay stores.
Epoxy and related supplies
U.S. Composites: Very good prices; this is where I've been buying my epoxy
System Three: I've used this brand with good results
West System
Galeton: Gloves and other protective gear
Marine Plywood
Noah's: I have purchased from these folks and was very satisfied.
Midwest Marine Plywood: Eagan, MN. A guy selling out of his suburban garage. 651-882-9704
Boulter
World Panel
Allied Veneer
Public Lumber
Edensaw
Plywood & Door
Racing Hardware
Sorensen: Hydroplane and runabout plans and kits, and racing hardware including steering gear.
Seebold: General racing hardware.
Safety
Lifeline: Racing life jackets and other safety gear
Security Race Products: Life jackets, etc.
Deist: Race and motorsports safety equipment, including life jackets
General
Defender: Discount boat and marine supplies
West Marine: Huge selection of boating supplies for all types of boating
Other
Surplus Center: I got my steering shaft bearing here, and my water pressure gauge.
Harbor Freight: Lot's of discounted tools and supplies, not necessarily top quality. I got my boat trailer from them -- comes via UPS in three boxes and bolts together. Also a good source for tarps.
Galeton: source for disposable gloves and coveralls.