What piece is this ARC-170 Starfighter cockpit? I got these with the ARC-170 LEGO Starfighter (archived):

 


I'd like to get more if possible. They are a part of the cockpit.
That looks like 30384 Windscreen 7 x 4 x 2 Round. In general, BrickLink is a pretty decent place to browse around for parts identification. Peeron is also pretty good.

BrickLink provides a comprehensive list of windscreens both plain and decorated.