Ron Hood sells some pretty stiff plastic bags (8 mil vinyl). A set of 5 bags , a "bag of bags" as he calls them, large and small sizes. 3"x5" and 6"x9".

I think the ETS survival kit bag by is 4"x5" IIRC from Aeromedix or possibly AMK.

Aloksak bags come in 4 sizes: 12"x12", 9"x6", 6.7"x6" and 4.5"x7". These are also odor resistant as well as waterproof but are not quite as rigid as the other two. The other bags may be odor resistant as well but aren't marketed as such.