Will Blick's cut an acrylic sheet to size for me, or do they just sell the sheet to cut myself (for poster frame)?

I would call and talk to the framing peeps there

They have a small variety of sizes, but the framing department does have the ability to cut plexi

Or, go to a framing shop. But call, call, call first.

I overheard that they have sizes no need to cut

I'm not sure. But this is worth checking out. They have a lot of stuff. Must be possible.