Just fill your gas lighter with Isobutane/propane mix from a commericial canister

http://www.msrgear.com/stoves/isopro.asp

you'll also probably need the Brunton fuel tool as well.

http://www.rei.com/product/708996

This should in theory give you an additional 11C colder working temperature compared to butane fuel.

Actually you may find that the working temperature of the torch using this fuel may even be slighter lower as the propane content will burn of first at very low temperatures enough to warm up the torch to get the isobutane above its boiling point of -11.5C.