Per Gareth Magennis:
I would really like a very small, cheap, air tank that I can charge every
now and again at the local garage (i.e. can be filled with a standard car
tyre pump).

Harbor Freight, as others have observed, is probably a good bet.

But how small?

If you want *really* small, start Googling "Paint Ball Guns". The
paint ball people who do not use CO2 use compressed air and the tanks
are quite small - yet hold something like 800 PSI.
--
Pete Cresswell