The cheap & easy way to find 13x18cm glass is to buy a clip-frame at IKEA... Since you're putting the glass behind the film, the glass quality doesn't really matter.

I originally intended using a HAMA spray-on sticky glue (sticky like the Post-it notes) for attaching the film to the glass plate for my 30x40cm camera, but forgot the spray can. So I took the stickiest thing to hand, which was blueberry jam! I would recommend orange marmelade if you have a choise. Just put a few small dabs on, and spread them really well with a finger. As everyone knows, jam stays sticky for months!

Format adapters: These are simple wooden "frames" made of very thin wood and painted black. In the corners there's a small strip of thin material to hold the plate in place. All of mine came in various old holders - including a 24x30cm to 18x24cm to 13x18cm to 10x15cm to 9x12cm set in one holder.

Film adapter sheaths: I don't think these are made anymore, but they turn up on ebay every once in a while. The largest ones I have seen are 18x24cm, I also have both 5x7" and 13x18cm as well as the smaller sizes. Note that 5x7" sheaths can be used in some 13x18cm plate holders, but not all. I have three seemingly identical plate holders for my 13x18cm plate camera - the 5x7" sheaths work fine in two of them, but the third one has an opening about half a millimeter too large. So sheath and film and all jump out of the holder when I pull the dark slide.

A lucky find on ebay has landed me a few 18x24cm plates to play with.