First off, for sheet film, if you're interested in FP4+, run don't walk to The View Camera Store to get in on the Ilford special order for FP4+ in this format, since the pre-order period is going to close at the end of the month, I believe. You can find their link in the "Sponsors" section. If you're not in the US, there are other dealers participating in the Ilford special order for custom sizes who can probably get the film, but The View Camera Store has been combining orders for this size in particular, so within the US or if you don't otherwise have a participating dealer in your country, it's probably easiest to use them.

Color you'll have to cut down and most likely process yourself, so unless you're regularly doing that, rollfilm may be easier.

You need backs made specifically for a 2x3 camera that uses a Graflok type attachment. Graphic and I think RB67 backs work, but not Linhof 23 backs.