Hello Richard,
I understand people's nervousness in using celly, but this is a lack of understanding of how to use them. In reality they are far easier to use than enamel paint, I can't comment on acrylics as I have never used them.
Obtaining them shouldn't be an issue as most car finishing shops sell them as they are still used in the restoration of classic cars. Colours are fairly easy, all mine are matched from RAL or BS, and as they mix well the together you can experiment with shades ( my southern olive is basically deep bronze green with a spot of brown added)
A couple of hours practice spraying coke tins and then maybe the inside of some angle will soon have you obtaining that wonderful 'pro' finish.
To answer your questions,
1 They do mail order, not sure of costs
2 I think spray cans are 500 ml, tins of celly can be bought as small as 500ml also. The problem with smaller amounts is that they cannot guarantee an exact match due to small quantities of tint. Half a litre is about a tenner, not sure about spray cans.
3 It's used straight from the tin, celly doesn't go off although it may thicken over time, just add a bit of thinner to the tin.
4. I generally go 2 or 3 parts thinner to paint, but it can be sprayed as thin as you like it just takes longer. I know Beeson used to thin his celly about 6:1 and gave his models about 20 coats!!
5 celly can be recoated immediately, but if you need to rub any dust out I would let it go off for about half an hour. Then I would spray over with neat thinner to smooth out rubbing down marks then add more coats if needed.
6. About 2 hours as a rule although some colours (reds) I leave overnight.
7 that depends on the pressure to wetness of mix and what I am trying to do. This is where the practice and experience comes in.
Generally tho with a 2:1 paint mix and about 15psi I would be about 2 or 3inches away, I would also advise a nozzle size of about .5mm to get plenty of paint down.
8. I just blow a bit of standard celly thinner through and give it a wipe with a bit of kitchen roll, that's enough. I will strip my airbrush down a couple of times a year and replace the seals, this is for me using it daily.
The most common fault when spraying celly is to be too careful, this encourages low paint to high air spraying resulting in a gritty finish as the paint is dry when it lands.
Give it plenty of paint and you will get a totally flat high gloss finish which is a joy to line onto, this is then matted down to your preferred finish with a lacquer coat but that's another story
Ian Rathbones book goes into spraying celly in more detail, we'll worth a read if you haven't already, also I shall be doing the painting and lining demo at Cleckheaton so if you are going come and have a chat.
Warren