www.mouser.com is an online electronics supply store where you can purchase just about anything and everything electrical. They have a huge selection of connectors, heat shrink, and sealing compounds you'll need for your project. There are no minimum order requirements and they're happy to take small orders form hobbyists.
Something to think about is the need for waterproof connectors. Your dealing with low voltage DC so a little water getting into the connectors is not going to cause any "shocking" problems. All you really need is automotive type spade connectors with the plastic shields to prevent accidental short circuits when in close proximity to other wires or conductive parts and Vaseline jelly. Just pack the connectors with Vaseline jelly, connect them together, and wrap with spiral wrap to protect them from damage. The Vaseline jelly will prevent oxidation of the wires and connectors.