I try to buy from HTG they stand by their stuff and they are cheaper than a lot of places. I didn't get a cord and they sent it to me asap. no questions asked. I also know of someone on here that had a problem with a bulb and they replaced it no problem. Here is there site, sorry I saved what I got which is 600 watt HPS but once you get there look around High Tech Garden Supply
Don't know if it is the cheapest but HTG Supply.
How about 150w hps complete for $20? You could use two.
It's just a basic industrial fixture not horticultural but will certainly do the job.
http://www.e-conolight.com/Product/EProductDetail.asp?ProductFamilyID=7&FGNumber=E-MT6H151G
You have to open this link twice to get the right page for whatever reason.