I get mine from NAPA. Its overpriced but I can see the store from my door.
Granted harbor fright has 3M BONDO brand stuff cheap!
If I remember right it was in the 20s for a gallon of real bondo..... I did not remember the price on the gallon jugs of resin .
I have resin for my friends pep stuff but frankly I prefer 2 part epoxy any day over resin as its less nasty and as a real glue can bond to anything ..
If you have a Lowe's store near you, they sell resin by the quart or gallon. 3m brand too. For the matting, Lowe's also seals the woven cloth, and if you want matting, you can order 30 yards for like $25
WalMart is where I found mine. If you have the WalMart app for your phone and buy there scan your reciept and you can get the difference in cash if it's cheaper somewhere else.