I’d try the agricultural land agents. (Kivells for one, covers most of the south west, but there’ll be local agencies across the regions. Eg., Nancekivell for Exmoor.) They’ll be handling farm tenancies etc., and would be most likely to be handling any smallholding lettings. And check with the county councils if they have any county smallholdings or small farms they let out, and where they advertise when they do.