Totally newbie, I just purchased a 9 x 12 Voigtlander. It is in great condition and can't wait to get into the field. Never shot large format before. It came with 6 original film holders. All still contain glass negatives. Can I recondition the holders? Can the be converted to cut film sheets? It seems like there would be a lot of wiggle room for film sheets. I have read that the external dimensions of 4 x 5 film holders are the same as 9 x 12. Should I buy new holders? Will any brand do?
You could just load the negative in the holder using the glass plate as a backing. I do this with a 10x15 plate camera I use. I'd suggest trying it out with an old negative, but you wrote it's your first time trying LF, so if you don't have one available maybe cut a piece of paper or some plastic to size, then try loading that, and check that it doesn't fall out when you pull the darkslide. It should work fine though.
I've bought some metal sheet to make some inserts, first I'm making quarter plate adaptors but then 9x12 as I have a few but need more. I've some other jobs more urgent but hope to begin next week or over Christmas,