There's a chain in the UK called The Works who do art supplies including quite good quality brush sets. I use their 0/0, 1/0, 3/0, 5/0 and 10/0 (retails for 2GBP the set) for detail work - instrument faces etc.
They also did flat brushes in similar sets, but I haven't seen them recently. You can't beat a good flat brush size 4, 5 or 6 for cockpit interiors. I usually brush interiors (apart from metallic) and airbrush exteriors. Quite often I'll use a big flat brush size 11 or so for Klear/Future or self levelling mixes.
I've got some good expensive Winsor and Newton etc brushes but I tend to use the cheaper Works brushes and bin them every so often.
I'm sure there'll be similar stores in US. Don't trust eBay though, better to see first what you are buying!
Sandy