It is not possible to buy them off the shelf. You have to build them yourself. To overcome this inconvenience people started to build them in batches. This saves money and time. At the moment I'm aware of several users in this community that are assembling proxmark devices. You can find them in the trade section. You can also start a batch build yourself, just try to gather in the trade section a bunch of people that want to buy it from you and invoke your local electronic-assemble-office.
new The user proxcat sells Proxmark related hardware from this webshop.