A good source for glass plates is down in Worcester, MA: S I Howard Glass. I use the 1.2mm soda lime glass. They do have a $50 minimum order, though.
You can find wooden plate holders on eBay, but sellers never know a film holder from a plate holder. The best 4x5 plate holders are Linhofs, but they've become terribly scarce. Be careful, however: you want the ones with the plate ejector (auswerfer) on the side. Look for an arrow on the front of the holder; no arrow = no auswerfer.
How many toes does your cat have?