Personally, I recommend T-Slots (
http://www.tslots.com/)
We purchased all of our extrusions, brackets, bolts, nuts, etc from them last year.
They have a huge variety of stuff that'll match 80/20 and will have anything you could possibly need, really.
The best part? They give 50% off to FRC Robotics Teams and offer FREE shipping once each month. When you're ordering, like, 100 pounds of extrusions, brackets, and bolts.. well.. that's a deal you can't refuse.
Plus, we ordered $2,000 of stuff last year that pretty much did it for our robot - but wait, the 50% took it down to $1,000! So yeah, T-Slots is amazing.