I ordered some custom laser cut plastic 2mm thick discs on ebay from a seller.
As for other monsters, these all worked well for me at correct sizes:
The Brachiosaurus doesn't fit perfectly, but 3 out of 4 feet fit on the base, and he's stable. Beside, he's supposed to be super long.
The triceratops is just a bit too big, but he looks nice and he stick out at a height above the player minis, so it's not troublesome. It's not a big difference anyhow.
To clarify, the triceratops, ankylosaurus and stegosaurus are huge, not garb, but it seemed relevant to the matter at hand.
Erik specifically said they don't intend to make dinosaurs, so it's a safe one to make do with ourself since it's unlikely to get made later on.