... I found some on ebay ...
I've also found sheet film adapters to use film in plate cameras, in 6.5x9cm, 9x12cm, 10x15cm, 13x18cm, 18x24cm and 5x7". That makes it a bit easier. And then there's "the jam trick" which Roger refers to, which I use for even larger films in even larger plate cameras.
Some of my plate holders came with format reducing masks, which lets me shoot any size plate in the 24x30cm camera. Since that camera has a minimum flange focal length for infinity focus of only 44mm, I'm not even limited to longer lenses!