I've seen 6x9 filmholders and even 6x9 Grafmatics, but not that fit a 4x5" camera. You would need some sort of adapter back for that. Better just to shoot 4x5" and crop or use a 6x9 rollfilm back, in my opinion.
mark - they are for the tk23, but i dont know if there will be problem using them with the mt45 , im not femiliar enough with master. any way, there is only one film in this format - the hp5+ (u can find it in b&h in ny) , which any way one of my fave films..lol.
u also can call the b&h and ask them because they do have the linhofs.
but then - u know, why should u, cause u have much better choice of film in the 4x5. if u want the 2/3 proportion u can crop - say 8x12 instead of 9x12.
what u also can have for the master is the slide-roll-film-holder from linhof, it is less expenssive as well than the roll-holder, which u dont need if u focus and work on the groundglass.
Watch out - 6x9 is not the same as 21/4x31/4. My Linhof 2x3 holders are for 6.5x9cm, with special inserts to hold the inch sized film.
I've never heard of a 6x9 sheet film holder for 4x5. The only holders I've seen are for 2x3, and these seem to vary according to which camera they were made for. For example, I have a no name 6x9 holder that is completely wrong for my Linhof, but (I believe) fits a Mamiya 23.