Shanda SD-900. 8 Eur incl. shipping from China.

There is one screw at one side...

...and four to hold the bottom plate (with rubber feet) in place:

Inside, there is an electromagnet, directly powered off mains. The "amount of air"-switch bridges an one-way rectifying diode. An metal arm with magnet glued on its end is driven by the electromagnet and drives the pump membrane.

Isolation distance is fabulous:

The pump mechanism consists of two chambers. An intake-chamber is normally directly connected to ambient, the chamber with the membrane got an input and an output valve so air is sucked in one way and pushed out the other way. For this model it is not possible to reverse the valves, so no easy modification is possible. It is possible to cut off the outlet flange and glue it onto the plastic pump body pointing towards the intake chamber. A 2mm drill helps to align everything while the epoxy cures...

and, well, it sucks: