The bonding putty can be bought at most quality retailers of boat building supplies... I know FGCI and Composites One both carry it. You can also make your own putty (my recommendation) by blending epoxy with milled fibers and cabosil. You add the fibers FIRST, get it somewhat thick, then add cabosil to get to a putty consistency. That will be VERY strong. I don't like buying the premade stuff because you almost always buy too much - plus you cannot control the consistency.
The other option is how I bonded my deck to the stringers: 5200. It is a very tenacious bond, and it is semi-flexible... so the deck has a little cushion on the stringers. Believe it or not, this is an accepted way to bond the deck - and used by a lot of folks.