Paintsniffer:  Preiser makes a fantastic set of USAAF pilots and ground crewman though all of them are standing. I recently used one of their 1/72 sets for a B-24.

http://www.reynaulds.com/products/Preiser/67003.aspx

Eduard makes a number of great sets of pilots and ground crew as well, though they're all standing.

If you don't want to modify standing figures I think you're pretty limited to resin which are going to be somewhat more expensive.

Not to jack the thread.. But does anyone know of any seated WWII bomber crew members in 1/48? I have a Monogram B-17 that I was thinking about populating.

Depends on what you're doing... WW1, WW2, Korea? Vietnam, Modern?  Male, Female? Fighter or bomber/transport? Leather, hard-hat, cap? US, RAF, Soviet, Luftwaffe, Japanese? Army? Navy? Civilian? Fixed-wing? Helicopter?

The numbers are rather limited, but there's plenty of kits that include seated pilots (although not as many as you'd think) and most can be converted as well...