MH v.s. HPS?..you'll find varied opinions and heated debates over that one.
MH produces more blue spectrum, which is utilized better during the veg state, but the red spectrum inthe HPS far out produces in flower. "IMHO" if you only plan on one light, go with the HPS. In side by side comparisons, I notice little difference in growth during veg, and HPS was superior for flowering.
Finding one locally?...I have seen 150's at places like Lowes/ Home Depot, (security lights), They take some re-wiring to "remote" the ballast, plus you really would need a different hood/reflector.
Take a sroll thru the "Yellow Pages", search for Hydroponic stores or horticultural lighting. You might find a store nearby.
I'm not a huge advocate of "online" shopping, but have heard good things about http://www.indoorsun.com/.
I discovered decades ago that night lengths LESS than 12 hrs. long triggered flowering.
When I started growing indoors I enjoed having as many as 5 seperate growspaces in the same house. I was able to try different technique's, and believe that a light cycle of 13/11 produce's the best buds.