Any gift wrap will do just make sure it is foil wrap and is water proof and has a shiny silver side to it. Found lots of it at my dollar store. $1 for 15 feet.
The mylar has to be perfect though, completely flat no stretches no folds or creases otherwise it could create hotspots. Flat white paint isnt as reflective but your not going to get any problems with hotspots
Check any party supply store or Michael's craft store or any craft store. Super cheap and widely available. I paid $3.99 for 15 sheets of 30"X30". I could have paid 1/2 that if I knew Michaels has a 50% coupon every Sunday in the paper.