Look a the sticky post in this sub-forum regarding the different 9x12 holders. Unfortunately there was no standard fit, although the actual plate then film size itself are standard.
I've bought about 20 plate holders over the past 18 months and you have to just take a chance they will fit sometimes. All mine have come from Germany or neighbouring countries.
There's usually quite a few on Ebay, but Ebay doesn't always map across properly so it's also worth checking Ebay.co.uk and Ebay.de just search for 9x12 in the photographic sections. The Ebay login is Global so it's the same User name & password regardless & a common "Watch this item" database, they all run off the same US servers in Denver anyway.
Welcome to APUG BTW