I know some of the chapters sell it, but I'm looking to pick some up between events. Are there any chain companies (I.E., Lowe's, Home Depot, etc.) that sell black corrugated plastic, or is this an order-only thing?
You would probably be better off with art supply stores or picture framing shops. Any of the Lowes or Home Depots that I have looked at do not carry it.
Tap Plastics carries it in many colors. Sells in 4'x8' sheets and they will cut it to any size/shape you want for a little extra cost, depending on the complexity of the cuts.
If using the plastic for a shield or anything else that might take a beating, I recommend doubling up the plastic with turning the "grain" 90 degrees on the second sheet for strength.
Nah, if it comes down to it, I can grab some from Jesse -- I was hoping someone know a chain vendor that I could check out locally so I could remake my shield before Deadlands.