I assume you mean underwater plumbers epoxy.Its a green stick with white center.You mold it together unit its a uniform color.You can get it at Home Depot or most any hardware store.
Can I make a suggestion, it you are going to use that crap, crazy glue the frag or rocks to it and the epoxy to the location you are picking, that stuff is really hit and miss. What I mean is sometimes it sticks, sometimes it don't.
Chris is right about the hit and miss. the rocks have to be pretty much dry, to get a real good bond. if you can get a good bond with wet rocks, it will dry with an incredible bond. if you cant get a good bond it will diy to be a semi-good bond. either way the rocks will stay. but, why would you want to bond all your rock together ?? you will have a very difficult time if you decide to re-aquascape your tank !!