Depends where and what you are looking for.
Crofts tend to be advertised locally (eg HSPC or equivalent or by local solicitors) quite a few crofts just don't get advertised. Best to know what area you are looking for and find out which solicitors are active in that area.
Farms tend to be sold by the big rural property firms (eg. Bidwells, Savilles, CKD Galbraith)