Not locally, though I KNOW our local highway department uses them for trash. I could probably stop in and ask for a few. Home Depot carries clear 55 gallon trash bags (we use those in our barn), which would make nice transpiration bags, but are no good for signalling.

I ordered them from here:

http://www.spectrumbagsonline.com/categories.php?cat=11

It ships in a quantity of 100 bags for $45 for the 55 gallon ones and 200 bags for $57 for the 33 gallon size. Adults would need the 55 gallon size, but the 33 gallon are big enough for younger kids. These aren't too bad a count considering they can be used for several things - maybe even garbage(!). I actually bought both sizes - 33 gallon for my son's Cub Scout den and 55 gallon for me & adults. Both work for other stuff , depending on the uses. For example, we use them to package my kids' sleeping bag and duffel for school trips and other uses when visibility or uniqueness helps.