The highest wattage HPS at my local Depot is 70watts. And as far as finding HPS on sale there, now may be a good time to look. When i bought mine they were on the same isle as the security lighting but on the opposite end (not near any other lighting fixtures, so look around, seems they dont want clearance items mixed in with reg retail stuff so you may have to search around a bit. And try not to ask an employee, they arent stupid,(i take that back) but they know why your after that light. Its commonly sought after by pot growers.
With all that said, i finally got a phat check and ordered a 600watt cooltube (both hps and mh) from HTG. So my 2 50watters are going to be EXTRA light for the sides in a few days I hope.