For what kind of internal overflow?
If it's open top, surrounded by water, and full of water, it doesn't need to be very thick. There's no pressure on it. I'd still go 1/4" or more. Gives you more surface area to bond, and less likely to crack over time.
As to where to get it, you're looking for cast acrylic sheet. Look for a local vendor. In the midwest, Regal Plastics is kinda the goto spot. They've got locations all through the central US.
I had my LFS guy, who deals with acrylic much more often that I do, build me an external overflow box for my new system, but I used H2Overflows inside. It's black acrylic, 3/8" thick, and has performed well for me so far.