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.

Ian