GE Polyershapes probably has the best selection and price. Go for the cast and not extruded. They carry Weld on as well. Go with #3 or #4.
I like 4 a bit better because it has a longer set time and gives you a bit more time to work.
If you havn't done this before you can go to Melev's site. He's pretty much made every kind of sump you can imagine and is good for ideas.
http://www.melevsreef.com/