With this size camera there's no point using sheet film, it is available to special order as part of Ilford ULF annual rum, and I think Wephoto sell it as well.

Pre-WWII Rollex backs are quite common second hand but beware there's a few different edge types you need the right one, and make sure it's 120 not 620. The big adavantage is 120 film isn readily available, the format's only marginally different.