Does your local car paint shop not sell it? Many do and it comes with a label saying it should not used for auto refinishing, only for that aeroplane you are respraying.
I recently bought a litre of cellulose (and the thinners) from Paints4U
Annoyingly, while they still do cellulose (in solid colours only), it looks like they've dropped 1K mixed colours from their range, which I'll need soon. Those other links will come in handy...
A lot of people have only a very small selection of 1K acrylic colours these days. The folk in my link (they are the Bedford paint shop) won't mix anything more than a basic range (from one swatch book).