I used Hellolights.com for DIY parts in the past as well as Amazon.com. Amazingly, Amazon beats other places frequently on prices and services.
Try this page for DIY parts: http://www.hellolights.com/t5fluorescentdiyparts.aspx
You should also check their retrofit system page as well: http://www.hellolights.com/t5-retrofitkits.aspx
It's often cheaper and easier to assemble an electrical fixture when you buy parts together. You can use the information on one site to check how much Amazon charges for the same thing.