First, its imperative that you determine what exactly type of light it is. Is it Mercury, Halide, HPS etc. You must know that first in order to make any kind of informed decision. Places like the Depot usually do carry both halide and HPS lights which is what you want. No mercurys, no quartz or any other types are worth your trouble.

The ones sold at the depot are going to be security lights, not growing lights. They will be extremely heavy and usually mounted vertically instead of horizontal. In order to utilize those to their fullest potential, you will need to take them apart, seperate the light bulb fixture part from the ballast, re-wire / extend the wiring and either buy or make a reflector to mount the light bulb part in. It's a fair amount of time and work but it is possible and the end result will be the same as a bought grow light if you do it right.

The one and only possible advantage to this approach, is if you do not want to have your name associated with an on-line order of a grow light or you do not want to go to a grow store. It's a PIA, but I did exactly this years ago when I had an indoor op for exactly those reasons. Be sure to pay in cash without any type of prefered customer card or the entire process is a waste of time as associating your name with an HPS security light is not much different than associating it with a grow light. The DEA and cops are on to this approach too. Be sure to use real heavy guage like 12 guage or heavier wiring and have a circuit breaker in the line. Solder all connections with a high wattage (60 W or above) soldering iron. Tape them thouroghly after soldering. There are plans out their for creatng your own reflector from sheet metal with common tools.

good luck