You haven't said where you are based, but for starters look for either PLA or ABS on this list:
http://reprap.org/wiki/Printing_Material_Suppliers
(The Reprap wiki also has pages explaining the pros & cons of each.)
I don't know what filament diameter(s) the Printrbot will support.
I apologize for not saying where I'm based, (US by the way). But that link is absolutely perfect.
According to the comments on the kickstarter page it'll support 3mm to start with and they plan on having a 1.75mm hotend available for $59+shipping by the time that all the kits are shipping.
I don't know what filament diameter(s) the Printrbot will support.