It's pretty hard to find lenses in Bergheil bayonet; I suspect they didn't sell very many accessory lenses, and most of the originals have stayed with their camera bodies. Searching eBay for "bergheil" will occasionally turn up something, and the high-end camera dealers like the LeicaShop sometimes have one (but the prices can be astronomical).

To make matters worse, I don't think the bayonets are interchangeable between 9x12 and 10x15, and there are a lot more 9x12s out there. On the bright side, those Heliars can be insanely good lenses. (Mine is one of the sharpest lenses I've ever seen, but it was originally bought by my great-grandfather, who would have been the kind of guy to cherry-pick the best lens available rather than just taking what he got. I don't know how consistent they are.)

If I were in your position, I'd ring SK Grimes and see if they can fab a bayonet mounting for the camera. If they can, you could have them do an adapter and be able to mount modern shutters. Finding an original shutter/lens for the 10x15 version is a worthwhile quest but will likely take a while, and you might as well be able to use the camera in the meantime.

It looks like the Lumiere Shop and Wephota, both in Germany, have a couple of emulsions available in 10x15, but the gain in area over 4x5 is marginal.

-NT