Home improvement stores don't carry them, but there are places where you can get them that will ship them. The two places that come immediatley to mind are Northern Tool & Equipment and Global Industrial.
That said... you may be able to score a couple for free (or a six pack) if you do a little homework.
Check with commercial electricians. Commerical tentants are always moving into buildings and converting warehouse to office. That usually yields a number of these fixtures. Electricians usually haul them off and sell them as scrap, so all you need to do is trump what they get in scrap.. which is probably no more than a few bucks, or a six pack.
Now... be careful here. These fixtures in buildings usually run off 277 or 480 volts, neither of which you can get in a residence. Many fixtures are designed to run off of the specific voltage, but many others have multiple voltages that can be selected ... usually 120, 208, 230, 277, and 480. These are the ones you can use.
To sound like you can walk the walk, when you talk to them, what you ask for is "400W Halide High Bays either 120 volt or multi-tap." They'll know exactly what you are talking about.
