You might get lucky and find some spare filmholders. I have been looking for 12x15 gear since I was given a camera in 1999 or so and I have only ever seen them for sale with cameras.

30x40 cm (which is very close in size) was supposedly a standard X-ray size, so you might be able to find some X-ray holders if you know the right skips to check out. It was also a standard graphics arts size, so you might be able to find a holder from a stat camera or something similar that you can adapt.

There are ways to make your own, especially if you can live with single-sided holders. Various threads in the archives here at apug discuss ideas of various levels of complexity.

But if you want to get taking photos quickly I'd suggest getting a camera maker like Chamonix, Argentum or Richard Ritter to make you a back you can attache to your exisiting camera and compatible holders to go with it. When I win the lottery (and lose interest in long lenses and long walks) I would like to adapt my camera to the 40x40 cm square format. The owner/front man for Argentum has some pictures of his camera and holders in this size on the Argentum website: