Sheet film comes in all types and sizes, although fewer types than once was the case. The common sizes are 5×4, 5×7 and 10×8, these being the size of the sheet in inches. All come in metric sizes as well, which do require different film holders. For example, there is very little difference between 5×7 inches and 13×18 cm, and some holders will manage to grip both; but in general, a 13x18cm will be too large for a 5×7 holder, and a 5×7 sheet will drop from a 13×18 holder when the dark slide is removed with the holder in the camera.
The older sizes – quarter plate (3.25″ x 4.25″), half plate (4.75″ x 6.5″) and whole plate (6.5″ x 8.5″) are still made, at least by Ilford in black and white. Half plate will fit into a 5×7 camera, and I have holders for half plate, 5×7 and 13×18.
Available to special order are other older formats, like 2″ x 3″, as well as the ultra large format sizes like 8″ x 20″, 10″ x 12″, 16″ x 12″, 16″ x 20″ and others. The cameras to hold these sizes are still made as well.
Film holders are relatively simple – all 5×4 holders are interchangeable. All you have to worry about are whether they are light tight. The “modern” ones are Toyo and Fidelity (not counting the rather more expensive wooden ones), Used from £5 to £10 usually; new ones from the usual suppliers. I have found Mike Walker (Walker cameras in north Wales) often cheaper than others for new holders, at least for 10×8 ones.
Quickload and Readyload holders are not usable now, as they were made to hold film in packets saving the need to load a film holder at all. Quickload was the (discontinued) Fuji standard, and Readyload the (discontinued) Kodak one. Holders readily (or quickly – excuse the deliberate pun) available, but the film is no longer made.
The alternative is Grafmatic. The holder is about the size of two conventional film holders, but it holds 6 sheets of 5×4 film in metal holders (septums) inside; a quick push/pull action after exposure slips the exposed sheet to the bottom of the stack and leaves an unexposed sheet ready to go. As fast as winding on a roll film camera. Coupled with a rangefinder style 5×4 camera (the archetypal press camera) you could point and shoot fast action. When buying, bent septums are the thing to watch for – plus if you have all six. Reputable dealers will have checked this.
Or you can buy roll film backs. These either slip under the ground glass (with a need to check that the particular camera has a large enough gape or the film back is narrow enough) or just clip in if both are Graflok compatible.
Moving up from 5×4, the point to watch out for in 5×7 has been noted above – 5×7 and 13×18 film generally won’t interchange in the other size of film holder. What can make buying unseen tricky is that often sellers don’t appreciate the difference, and in the case of Fidelity branded holders, I have found that they can have “5×7” clearly embossed in large print on the holder, with a much smaller 13×18 underneath. I did once buy a 13×18 holder where the price sticker had been placed over the spot where 13×18 was printed, making it apparently a normal 5×7 holder. As I use both sizes, this wasn’t a [problem except insofar as I had one more 13×18 holder than I expected!