Doug:
For ‘shapes,’ Evergreen or Plastruct are the best bets. Lots of hobby shops and other dealers carry their lines and sell smaller quantities.
For sheet materials, I use TAP Plastics. Their San Leandro, CA store is managed by Russ Miller. Russ is a long time skilled LS modeler, the current president of the Bay Area Garden Railway Society, and is very knowledgeable in all areas of plastics, fabrication and joining. His store has laser cutters that will produce virtually anything that can be cut from a sheet of plastic for very reasonable prices. Lots of folks use Russ to supply building and car parts for custom projects. He is also an expert in things like molding materials and other specialized stuff. He is very accommodating to fellow modelers.
Russ can be contacted at his store:
http://www.tapplastics.com/shop/locations.php?lid=9&
Hope this helps.
Happy RRing,
Jerry