If I recall the GE Silicone I, 100% Silicone for Doors/Windows/Attic, is the one you want. Anything with MILDEW blockers is NOT reef safe.
Just do a Google search for "Glass Company <Your City> MA" and you'll get lots of hits.
From prior experience I can tell you you're better off just buying a tank from someone or waiting for the $1/gallon Petco sale to come back around. Glass prices are kinda crazy, not to mention you have to do all the work, and then hope you did it right.
What size frag tank are you thinking to build? Did you figure out the glass thickness and sizes of the panes you'd need? Depending on the size you might need to brace it somehow (euro-brace, etc.). And, when/if you get a quote for the glass, make sure you get the edges polished so there aren't any sharp edges.
If you're going to build the tank yourself you could look into "Aquarium Safe" silicone. More Googling will find vendors of that. People also suggest a specific type of silicone for larger tanks. I don't recall the name, but it was supposed to bond stronger.