It really depends on what you’re making.
I overwhelmingly prefer 1/8" for most of what I’d make with it. Sometimes I go down to 1/16" when I am making something like a frame where it’s meant to be the “glass.” I almost never use 1/4", but I’m making smaller things generally.
I think the best place to buy general acrylic is at a local plastics supplier. Prices tend to be lower and you don’t have to worry about shipping. You may need to cut it down or pay them to do it, though. Other than that, yeah, a search will give you the best options. I have a thread about “Fun acrylics” and that has a lot of options. There have been a lot of places crop up recently like Cerulean Tides and Waii Supplies (if you Google either, you’ll find them easily) that cater to laser users. They are always trying to find cool acrylic options.