My transit tickets have a magnetic strip, and are regularly getting demagnetized. I've eliminated every magnet I can find, but despite my best efforts, I cannot track down the source....so have to assume its coming from outside the house. I'm looking for any cheap/commonly available material to make a ticket holder that will protect my tickets. I'm already aware of the expensive stuff (like metal alloys). Any ideas?

It will have to be a magnetic material, for example iron. Before aluminum cans there were cans made from iron/steel which would be ideal. However, such cans are rare now. You might try some iron strips embedded in plastic to make a card holder.\

http://www.mansionschools.com/electrode-iron-strip-flat-1561-4-power-physics-electricity.html?channelid=GoogleAdwords&gclid=CJzBrbKonrgCFbTm7AodQxAAaw&captchaChallenge=Yes

RF protective sleeves might afford a small amount of help.