Chicago Airbrush Supply, who have yet to fail me. They rock.
This time it's a Badger 200GF, a single-action gravity-feed internal-mix airbrush. I bought it with the Fine (F) head/needle, which will do lines down to less than a mllimetre thick.
Paint flow is adjusted via a knurled knob on the end of the brush, while air flow is controlled directly from the compressor's regulator.
Thus far, I've only done a quick test-sheet in watercolour on 300gsm laid paper (see below), but already I can tell that this airbrush and I are going to get along swimmingly. It's my favourite brush of all that I've tried so far, and even though it's a humble single-action, I doubt that that will often be any kind of issue.
Even at very low air pressure (I went as low as 6psi for my test-sheet) its spray is fine and even, and its lines are pretty even and regular.
It cost me about $45 from Chicago Airbrush Supply, which I consider to be a very good price indeed.