try calling your lumber yards some places can order sheets for you. I live in NY and one company that was used at a shop I worked was Premium Plywood Products, Inc. Home Depot might even sell it or Lowes. Are you goin to glue the plexi together or make frames? The process to glue plexi takes time and cost a good amount of cash to do so. For a strong bond you want to use a UV machine to do the bonding ( those are not cheap ) I have never used them but I worked for a company that makes display cases for museums and the bonds MUST be good... Good luck on the builds I would try to come up with a frame made from plastic..

Cheers