Almost everything (apart from Okoume Plywood) for boats being built from plans can easily be purchased online. Please feel free to post any of your favourite online retailers, along with materials they sell. I'll be adding my own favourites to the list shortly.
My two favourite online retailers for boat building materials are Raka and Duckworks. Almost all composite materials required can be found at Raka, and their prices are extremely reasonable. Duckworks sells both composite materials and chandlery. Any sort of obscure boat knicknacks - from rope to anchors to blocks - can be found at Duckworks. And like Raka, Duckworks caters to the budget-conscious builder, and has great prices. Between Raka and Duckworks, you should be able to get everything you need to build a small boat (apart from the plywood.
www.raka.com
www.duckworks.com