Fomafoto sell 9x12 film and postage to the UK is quite reasonable & fast, not sure if Matt at AgPhotographic has 9x12 in stock.

You need film inserts for the plate holders ideally, I have been trying to get some new ones made (in the UK) but my usually source for metal parts doesn't work with steel thin enough. There's nothing complex.

Your plate holders appear to be reasonably standard but there are rather a number of variations. It's rather hit & miss buying them as usually sellers don't show the dimensions etc very well.

Great camera, looks very nice & fairly unusual.