Hello, and I hope you had a good Christmas too.
On a sad note, you will find it impossible (don`t hold me to this) to find Model Master Acryl paints here in the UK. On a better note, you will be able to find Polly Scale acrylic paints, which are made by the same company as Testors (RPM company)
Hannants in the UK sell Polly scale.
On the clogging of the airbrush tip, you will find most acrylic paints will clog, so the best action is to keep wiping the tip with a cotton bud (Q tip to our cousins across the pond)
soaked with acrylic thinner. I find that Tamiya paint doesn`t clog as bad, as long as you use their brand of thinner, which contains alcohol. Failing that, Gunze make a product called Mr Levelling Thinner, which is supposed to stop the acrylic paint drying too fast. Keep and eye on ModelDesignConstruction in the UK as they sell most of the Gunze range.