Hiya David,
I have no experience of using humidifiers but I successfully cultivate a variety of cool-growing orchids in an unheated greenhouse during the summer (May to Oct) then into the house for the winter. Genera grown include Masdevallia, Pleione, Laelia, Dracula, Coeologyne, Dendrochileum, Dendrobium and Pholidota. In summer the typical day greenhouse humidity ranges from 40-70% and at nights almost always 95%. Damping down during hot weather lifts the day humidity by about 10%. The orchids do fine over the winter in the house with humidity approximating 40-50% If you are heating your greenhouse then a humidifier might be useful but it goes without saying that air circulation must accompany high humidity otherwise fungal leaf spotting will appear. The YouTube channel “Micks Masdevallias” is run by an English gent who uses a humidifier (hydrofoggers) all year so you might be able squint and see the manufacturer! Sorry, I can’t be more help and I may be wrong but I can’t think of anyone in the society who has discussed humidifiers.
Can anyone reading this provide David with a recommendation?