If your not worried about presentation, 1 or 2 kitchen trash bags work well. You can also use a vacuum to suck out all the air in the bag to make it as small as possible. Getting a 170 in a medium flat rate box is usually not a problem and anything bigger fits in a large flat rate with that system.
www.uline.com the gusseted or flat poly bags would work although they sell them in bulk for the most part.... I'm guessing if you are looking for the thick clear bags this would be a place to start, that or a local shipping company ups store etc.