Some companies such as Northern, Arriva Trains Wales and Scotrail appear to have local instructions telling guards not to sell them and giving reasons such as the boxes (where applicable) won't print or not to sell them to prevent the risk of fraud but it's perfectly possible for them to be sold from an Avantix Mobile ticket machine.
Ironically 158757 bought one from Virgin Trains at Manchester Piccadilly where the boxes printed incorrectly (following the introduction of their new TIS) and he bought a new one onboard ATW and applied for a full refund on Virgin Trains bought rover. In that instance I think he got 4 Date boxes for a 3in7, if i recall correctly .