I've seen pictures of these old school CO2 pumps. Never seen one stand-alone for sale. It would be a nifty item to buy.
I bought a Pennine from this seller a few weeks ago,
he has another one with a few hours to go.
more details here on the Pennine.
You need to source 8gm non-treaded co2 cannisters, and they appear to be available from
cake equipment suppliers