How hard is it to come by 2x3 sheet film and why would one want to use it over 6x9 roll film? Couldn't someone just use a ground glass back for focusing (especially with the bellows out on a Super 23 for macro work) and then replace it with the roll film back to expose?
Sheet film choice is a bit limited, even more so as there are two different, incompatible nominally 2x3" a.k.a. 6x9cm sheet film formats (of the film, the outer dimensions of "International" holders seem to be the same). Foma, Ilford, Fuji and a variety of re-cutting/re-branding third party sellers have some. In colour there seems to be only Fuji NS160 in 6.5x9 left.
Mamiya holders are not "International/Graphic" standard, but a proprietary slim variation. Accordingly, both "International" and "German/Falz/sheet metal" type won't fit the Mamiya ground glass backs, you need the Mamiya ones. However, "International" holders will fit the "G" (for Graphic) back on the Universal - and there also are Mamiya branded international holders (for G back press cameras and the RB67), to add to the confusion. The Mamiya Press holders are excellently made, contain a film sheath and tend to be much more affordable than other 2x3 sheet holders with a matching G back.
Watch out which holders you have, there were two "6x9" film formats, European/Japanese 6.5x9cm (62x88mm) and US 2 1/4x3 1/4" (56x82mm) - Mamiya had "type J" holders that can hold either (or rather, its smaller size is a slightly wider "eighth plate" 2 3/8x3 1/4", but 2 1/4" will fit the sheath as well) plus glass plates, and a "type A" which can only hold 2 1/4x3 1/4" film and 6.5x9cm glass plates. Beware, in Europe, the smaller film for A holders can be hard to get, while the holders, thanks to ebay, now sometimes turn up there - you can work around with 6.5x9 film using a spare glass plate as a pressure plate behind the film (if you have some glass plates around, that is).
It is possible to switch between GG back and a roll film holder - but it won't be quite that easy on a light tripod, as the attachment requires some force. On the Universal, Mamiya themselves recommended the more easily interchangeable "P" (for Polaroid) ground glass back for that purpose rather than any of the cut sheet film ones.