I found them at Meijers Camping Section too. They are often sold as Emergency Blankets too, or Camping Blanket. I have seen them at Walmart too, and in both stores, they run less than $3 each. They are quite big, and I use them in boxes that I make as kitty shelters for our local Feral Kitty Colony at the grainmill at the edge of our town. I use VERY heavy black vinyl (found at Home Depot in their paint dept. with the disposeable drop cloths) Also, duct tape, styrofoam sheets (also found at Home Depot in the insulation aisle) and then heavy cardboard boxes. I cut the styrofoam sheets to the size of the "floor" of the box. Then put some newspaper sections (they can be removed easily, if needed, and I check the boxes daily, but if you can't check the boxes daily, don't use newspaper!) and I put the Mylar, also cut to size, in between the top newspaper layers. I went to Salvation Army Thrift store, and asked for woolen rags, or pure woolen clothing, I could "piece up" into woolen rags. They gave me a HUGE bag of them!
I put them in on top of the newspaper and I also check them daily, but pure wool, wicks moisture away. You can also attach the Mylar on the "walls" of your box on the inside to help reflect heat. I then close the box and cut a hole in the front, for a door, then use duct tape to keep it all in place, like how I want it. Then, I cover the entire outside with the heavy weight black vinyl and and use duct tape to keep it all together. I used these all last winter, and they are in a protected place at the grainmill, with 3 walls and a roof. They are still VERY useable this year, and I just took all the woolen rags out, and replaced them this past weekend, and added a few new shelter boxes.
Here's a pic of it, and my little KatyKitty is "testing" it out!