It's not currently available where I am (NE US) either. My best guess is that it's another casualty of supply line bottlenecks, etc.
Depending on your enclosure design and framing, would it be possible to use another material temporarily, and replace with PVC when available? I haven't checked prices, but these come to mind as possibilities...
ACM (pretty expensive)
Coroplast
Acrylic/Plexi
Glass (less expensive than acrylic)
Hardwood plywood (will need sealing)
Luan (will need sealing, but usually cheap)
Ceiling lighting panels (only come in 24 x 48 nom) really cheap
Still thinking...
