I'm always leery about using auto thinners on plastic. Some of them are very aggressive and I don't want to melt down something I put like 40 hours work into.
Don't worry about ENAMEL reducers (the trade-name for enamel thinner)... They won't attack the styrene.. It's LAQUER that will attack the styrene, and agressively.. However, laquers CAN be used on plastic models over an enamel-based primer, but it's not something I'd recommend anyone try without practice on several models that they don't care about... Any overspray on un-primed styrene will casue it to soften.. Best just to forget about using automotive laquers.. I used them years ago when I was building cars, simply because the color selection huge, and I was in an auto-body repair course and the primer & paint was "free"... I melted a few car bodies before I got the application process down right..
If you do pick up a laquer by accident though, you'll know immediately that you did as soon as you add it to your enamel paint.. The paint will almost instantly curdle in the mixing jar, so it'll never hit the model through your airbrush....
But I don't see a lot of enamel paints in local craft shops
Nope, and you won't.. However, some acrylics airbrush beautifully when reduced properly, and dry dead-flat... Which isn't always a good thing.. If you apply your base colors witha gloss paint, you can skip the clear gloss step for decaling... Just let the paint cure and start applying them... Then continue the weathering process with your washes, then flats, then dry-brush/pastels, etc.... Whatever you do..
Oils can be reduced as well as enamels, and applied with an airbrush, too... Tempera paints from the craft-stores make excellent sludge-washes, as well as allowing you take off any mistakes with a water-dampened paper towel... I use tempera paints for washes and stains almost exclusively, and dry tempera makes an excellent weathering powder... All you need in tempera paints is the basics-ie; Red, Blue, Yellow, White, and Black... With the primary colors, and the B & W aint, you make any shade you need if you're familiar with the Color Wheel, and can mix the primaries into second and tertiary colors, in which you'll find your browns and other "earth" tones...
Here's a link to the google page for the color wheel..
http://www.google.com/images?hl=en&source=hp&q=tertiary+colors&aq=2&aqi=g10&aql=&oq=tertiary&oi=image_result_group&sa=X
I know it all sounds kinda tedious, but it's one way you can take back some control over how much you pay for paint...