You're talking about a Graflex RB? The 4x5 models are hardest to find because they're the most popular because 4x5 film is easier to get and cheaper than either 2x3, 3x4, or 5x7. In the US, at least, 2x3 and 3x4 are available only in black and white, and only in a couple brands (Fomapan/.EDU Ultra and Ilford). 5x7 has a few other stocks available, but I'm not sure how many of those are NOS/expired.