By far the best pricing I found on rack was here
http://www.stdsteel.com/gr_stock.htm and I had zero problems with the purchase. But you have to ask for pricing and shipping and send a check, no credit cards. I have heard they have gone to per piece cost on shipping based on two shipping zones lately. I bought a gear blank from McMaster that's 12" long that I intend to make my own pinions from. Lot's cheaper and I can get the 1/2" bore that I need that way (I sure love my 12x36 lathe!.) I went with the 20DP and 20 tooth pinion but have yet to build. I've gotta finish the mill first.
You need to check out the MechMate at
http://www.mechmate.com/ and also be sure to scan the forums. They've already got a motor/rack/pinion combo worked out. Plenty of info and it's free.
Disclosure: The preceding two paragraphs were flagrant cut and pastes from previous posts I made.