A Word of warning:
I purchased extrusions from McMaster, based on the 3DR BOM attached (actually, it references a http://misumi.com source). I also purchased the parts printed by Wardtechprinting.com…the profile widths are slightly different!
This is in no way a reflection of either source, it’s just a comment so that you can learn from my experience.
Luckily, I’ve got a mill, and an end-mill bit that took just enough out of the center of the groove to make everything happy, but all pieces aren’t ‘parts swappable’, they often take a little massaging to work.
(The part on the left fits, the part on the right doesn’t)
https://drive.google.com/file/d/0B09sUvvteXCBX2ZxT0pSb1c1VFU/edit?usp=sharing