Crystal apple and other round types are just a form of the ridge cucumber and will grow rampantly under cover. Usually one plant in the greenhouse will give you enough of these to give you an abundance, and even make soup with them. Our main problem was picking them before they got too large and slightly bitter.
There's another little round cucumber called a Carosello or 'melon cucumber' which comes from Puglia. I think you can get seed from Seeds of Italy.