I noticed about six months ago I was having problems with some of my credits cards not being read by the machines at the stores. I called my bank on one and they said it probably got demagnetized and sent me a new card. It started happening again, so I picked up these sleeves on eBay about 4 months ago, 15 for $10 delivered, end of problem. I see he went up a $1, now $11. There are several sellers on ebay with different designs and prices. They even included bigger ones for a passport.
15 RFID Blocking Credit Card Passport Holder Case Cover Safety Sleeve Protector 849287037046 | eBay