Don't know about a "best" site, but here's a couple:
www.classicgamecreations.com
That's John Dondzilla's site, homebrewer of Vectrex games. I noticed several months ago his homebrewed Colecovision and Odyssey2 games were no longer being produced, and the same went for most of his Vectrex games recently, everything's available on emulation only now, pretty much...probably due to a lack of Intellivision cartridges (he modifies them into being the casings for his Vectrex games). There's a link about some place that has whatever games of his are still in stock, though.
www.classicgaming.com/vectrex
That's Spike's Big Vectrex Page...it's HUGE, actually!
That alone should keep you busy for a while