Does anyone know where I can get the oil filter intermediate flange gasket, part number 021 115 359 A?

I can not find it available from any of the usual suspects.

I need one ASAP.

It's a PITA part to find; I usually steal it from an engine gasket kit.

Seems there's one on eBay right now

https://www.ebay.com/itm/VW-TYPE-4-ENGINE-O...IP/193892993620

Can always buy approprate thickness of gasket material and make your own.

Gasket Punches come in handy in a pinch:
https://www.amazon.com/TEKTON-6588-Hollow-P...la-347787533660

I should buy a set of those, just to have around.

Hard to find. I got one from Bughaus albeit when ordering a few other things - pretty sure it was left over from a gasket kit.

Yep, the one on ebay mentioned by @GregAmy is from bughaus in Tulsa.

I ended up calling some VW specialist shops in Phoenix and one of them had some sitting on a shelf.