Where can I find glass or plexiglass???

My hubby and I found, well, frankly I don’t know exactly what it is. But I want to make a small table out of it, so my question is, where can I go to find a round pane of glass approximately 18 inches in diameter? Is there somewhere I can go where they will cut it for me? It doesn’t have to be glass, plexiglass would work.

Also, any idea how much something like that costs??

I appreciate the help!!!