I don't advocate the itty bitty canisters. if I'm carrying it I want enough for a good Seargent Hugh type hosedown, not make some central park mugger's eyes water a bit. I use this:

http://www.mec.ca/Products/product_detail.jsp?prd_id=845524442621480

It rides nicely on the side of my pack. yes I can reach
that in a hurry:



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