If you're using plywood then you'll get that from any hardware store.
As for the Resin... Epoxy or fibreglass will do. I'm also having problems getting me some decent resin. I get epoxy from a hardware or marine shop but it's expensive....
Try any surf shop... they might stock fibreglass resin (as surfers need it to repair their boards).
If you wanna use a surf blank instead of plywood then a surfshop may sell some.
Where I live in Auckland there is a place that sells surf blanks AND resin but as yet I haven't found the location of this place so I'm also still looking. 'aklbob' who I kite with quite a bit doesn't know the location either.
Anyway check out surf shops, hardware stores, marine shops for all the stuff you'll need.
What ever the cost it'll be a lot less than buying a new board.... It would cost me less than $100 in NZ to make my own wakeboard.