Where would I buy a round base made of plywood (or where to go to get plywood cut in round shape)? I need to make a 3D dinosaur and want to put a pipe going through the cake but want it secured down to the board. If not plywood for base, any suggestions for this type of design? Thanks!

Home Depot & Lowes will cut down boards for you but not into circles. You can buy circle pre-cut wood but it is usually a higher grade/higher price.

Maybe a lumber yard? Or do you know the shop teacher at your local high school who might do it for a few bucks? Or a carpenter? Or cabinetmaker? Or a hobbyist woodworker? Can't hurt to ask anyone who owns the proper power tools.