I have never bought it before. It's on ebay but where should I expect
to buy it in a town - high street, industrial cleaning agent
suppliers, car parts factors or..?

If you are unfortunate enough to live anywhere like here then try a
local nail bar: they get through loads of the stuff as nail varnish
remover and I've seen them[1] with 1 and (I think) 5 liter bottles.
Online prices seem to be ~£5 a liter:

[1] when passing - I can't afford to get my nails done at their prices!

--
Robin
PM may be sent to rbw0{at}hotmail{dot}com