the International Orchid Register does not list any species in the Dinema genus. Since this was a Laeliinae group it appears to no longer exist with the reclassification of Laelia.
RHS list Encyclia polybulbon as a natural hybrid and not a synonym. After it was changed form Dinema it was classified as an Epidendrum and then changed again to Encyclia. I usually see it with name tags still using Epidendrum.
It seems to be for sale everywhere this year.
---------- Post added at 05:08 PM ---------- Previous post was at 05:01 PM ----------
I have to apologize since a search of Kew Monocat list shows Dinema polybulbon as the accepted name and Encyclia as a synonym Just shows you can not trust even the official registrars