Many of the online sites will get you in the $30-40 range for resin, but then it is about $10 / gallon to ship it. Cloth is a bit more economical on the shipping side.
I prefer to go direct rather than through Amazon. iBoats sell some supplies, so please do check here for things that you will need.
U.S. composites and the one WoG just mentioned are places where I have purchased resin and glass with good results.
I was at West Marine the other day and their resin was $60/gallon
You could also do a google and yellowbook search for any places within driving distance, as that can save you shipping, but may pick up sales tax, depending on the exact situation.