htgsupply.com they seem to have the best prices on lighting systems, and if you call them they usually give you a better deal i got 15% off my 400w hps from there when i called.
I would say that if you are only going to buy 1 ballast, I would buy it locally.
I had a ballast that went out in the middle of a grow. The company handled the last problem very well, but it still took a couple of weeks turn around due to shipping.
I now have a ballast from a local shop, as I was in the middle of flowering and needed to go buy it that day. Ask your local hydro guy how they would help you if you had a problem in the middle of your grow.
Finishing your grow may be something to consider. Most people only think about the price once, the day they buy it.
spend a little more money to get a glass sealed air coolable hood. HTG online or local hydro shops have em. 400 watts still put off a lot of heat in a small space. your being happy with the product is worth the extra money. Do it. get the nicer bigger one.