I have not used a 2 1/4 x 3 1/4 Speed Graphic camera or it's associated film holders, but I'm sure that it can't be that much different from other 2 1/4 x 3 1/4 equipment.
As others have said 2 1/4 x 3 1/4 inch sheet film is not the exact same size as 6x9 cm shots that you'd get from a roll of 120 format film. They are pretty close, but I have my doubts that sheet film cut to 6x9 cm would fit into a 2 14/ x 3 1/4 inch holder.
There is at least one film manufacturer that still sell sheet film in the 2 1/4 x 3 1/4 inch format, Ilford being the primary one. I've seen that size sold at Freestyle Photographic Supplies online. I'm not sure if that's a standard size, or if that's one that they only produce during their occasional special order period and Freestyle has stocked up. There's also their in house Arista brand in that size, which I believe is manufactured by Foma.