I was looking for detail parts and complied a list of sources for my own use but thought I would share. Please add to the list.
G Scale Scratch Builder Parts Resource
Precision Scale Co. http://www.precisionscaleco.com/
Brass, nickel silver, and plastic detailing parts for steam, diesel, rolling stock
Ozark Miniatures/Hartford https://www.ozarkminiatures.com/Scripts/default.asp
Detailing parts for steam, diesel, rolling stock as well as trackside.
Larry G Scale http://www.larrygscale.com/
Trackside accessories, figures, diorama scenes
Mr. Details/Shapeways http://www.shapeways.com/shops/mrdetailparts
Trackside accessories and diorama
Trackside Details http://www.tracksidedetails.com/
Mostly brass detail parts for steam, some diesel and trackside stuff.
Grandt Line http://www.grandtline.com/
Building details such as windows and doors
Wiseman Model Services http://www.locopainter.com/store/category.php?id=46
Small selection of trackside details
Bachmann Parts http://estore.bachmanntrains.com/index.php?main_page=index&cPath=66_69
Many of the detail parts for Bachmann locos and rolling stock are available to purchase individually.
Accucraft Estore http://www.accucraftestore.com/
Many of their parts are available individually