Never done the dumpster thing, I would also fear broken glass or whatnot...if it came down to that I would rather pay the $12.99 for a case of empties from LHBS than free disease from the dumpster.
Otherwise, I just got the word out to friends, family, and coworkers. My father even took some of my homebrew to his neighborhood 4th of July party. All the neighbors are going to start saving bottles. In addition, one of the neighbors had brewing equipment, C02 tank!!!, which he offered for free. I will give him plenty of homebrew for it anyway.
I'm brewing 25 gallons this weekend and I'm not worried at all about having enough bottles, there are enough flowing in from everyone that knows me.
I also have kegs for backup, beer storage is no issue here lol
I guess my long winded answer means just one thing, make sure everyone around you knows you need them and you should have ample supply.