Hmm...
LEDs, drivers, optics:
www.ledsupply.com - US
www.cutter.com.au - Australia, more choice
Heat sinks:
www.heatsinkusa.com - US
Arduino, mechanical bits, etc:
www.robotshop.ca - Canada
I'll be building an LED rig this summer, using Cree MC-E whites and XR-E blues. I have yet to decide which driver will be the best for my needs though, so I'm still in the research phase.
I'm actually considering building a plant growth rig with XP-G whites, partially to see how these new LEDs are and partially because I don't have enough window space for all my seedlings... maybe I'll build that first and put it on my 10 gallon when the seedlings move outside.
Either way, hope it goes well for you... can't wait to see how it works when you're done.