Home Depot and lowes sell street lights, but they are only like 70watts. Go on Craigslist in your area and check to see if anybody is selling them used. Check to see if you have any light bulb stores in your area. Hydro stores carry them. If all else fails than order off the internet. If you are afraid of using your card than find out what the total purchase price would be for the one you want then go to WalMart and get a prepaid Visa card for that amount and use that to get the ballast.
You can get a 100W MH street lamp from lowes for $80. They have a lot of MH and HPS bulbs at Lowes, I've bought 400W HPS bulbs from there before (a lot cheaper than online and grow shops, too) but they don't really sell the lamps to go along with the bulbs they carry. I buy my lights online cause online's almost always cheaper than a grow store. At last check, HTGsupply's 400W HPS system (Ballast, bulb, cords, relector) is still the best deal around at $120 + s/h.