sorry, cannot resist - here is mine Linhof Bi with 5x7 and 8x10 backs.
View attachment 245585 View attachment 245586
Personally I'd rather search for decent holders - either 5"x7" or 13x18cm but most certainly search for modern plastic ones - and then buy film, but I shoot Ilford, which is readily available in both sizes. 5"x7" film should fit into 13x18 holder, but not perfectly (will dangle).
BTW, Linhof 13x18 holders have a pressing mechanism so should be ok to use 5"x7" film. But those may be too large for a field camera - e.g. they are too fat and do not go into my Rittreck View standard 5x7 back. But one could ensure if film is in via special checker and expose a number into the negative - there is a small window at the bottom where you can put a number. So they have nice little features!