I have been lucky in picking up 13x18 cm, and half plate film holders, primarily because I think some owners of 5x7 cameras have not considered them as 5x7 holders. Film formats I can either get directly, or trim to size in the darkroom.
PS. Oooops, should I have really said that. Actually, on APUG people probably realise that, but I think on the big auction site, they can slip past!