Save up your Sam Adams bottles. You can also go to a bar and ask them for empties. They will look at you funny but they usually let you pick through their dumpster. Make sure they are not twist off bottles as your caps won't sit right.
The experts say to use only brown bottles, but I've used green ones as well with no adverse effects. As long as you store them in a dark place it shouldn't matter. So, I've used Grolsch bottles, Sam Adams, Weissbier bottles (true pints), Guinness, etc.
I also bought a case of bottles - it's nice to have extras so you don't have to be a tool and ask your friends to return the bottles if you give some away.
If they still have labels, soak them in hot, soapy water for an hour to get the labels to peel right off and break up any caked on sediment in the bottle. Then, run them through your dishwasher on a sanitize setting or hot rinse and dry (without detergent). Inspect them before bottling your beer to make sure there isn't anything unsavory still in them.