Just a word about "standard" and "non-standard" film holders. Modern cameras take slides like Fidelity, Toyo, etc. which if given a name at all are called "blockform" slides - they have pull-out sheaths to make an exposure, hinged end flaps to load film but otherwise have no moving parts.
Earlier cameras, particularly Engish ones, had a type called "bookform", typically with wooden "roller-blind" type sheaths which pull out far enough to expose the film but do not pull completely out of the holder. Your camera seems to have these. The key feature of these holders is the two clips which, when released, allow the holder to be opened right out via hinges at the bottom to allow film loading (in many cases they will need a film insert in order to accept sheet film). "Bookform" slides may be "non-standard" today but were standard 100 years ago in England. You do therefore have some chance of finding some more, but you should try before you buy to make sure they fit.
There is also a non-standard wooden type slide which I have only seen on Russian cameras (particularly the "FKD" brand) which looks like bookform but has no hinges and loads through the opening created by pulling the sheath out. I have an FKD 13x18 cm, which is the most horrible view camera I have ever owned. The sheaths will not pull out of the slides smoothly, despite the application of candle wax and every other traditional filmholder lubricant, nor will numerous other alleged moving camera parts actually move.
Should you wish to make a 4x5" back for your camera, let me know. I have several which would only need a square wooden adapter panel to be made to fit your camera.
Regards,
David