If your buying flips you may not see the word Mylar because Mylar is not really a material, it's the product name originally given to things made from Polyethylene Terephthalate. Obviously Mylar is easier to say but if you don't see the the word, look for Polyethylene Terephthalate. You'll also want flips that are dust and oil free. The flip that can harm coins are flips that have plasticizer in them to make them soft---also called PVC or "vinyl" flips. Under certain conditions the plasticizer can leech and harm coins.
Usually Polyethylene Terephthalate--"Mylar" flips are hard flips due to the lack of plasticizer. I believe there are also PVC/vinyl soft flips that have a "Mylar" lining to them, but I never tried them.
*Usually* flips that are soft and inexpensive have plastizer in them. If they smell like a new shower curtain and/or cost about $4 for 100 of them, that's a sign that they are PVC/vinyl flips. They are not only inexpensive but real easy to work with because they are soft and flexible--another warning sign.
Usually flips that are hard and cost about $16 or $17 per 100 are "Mylars" or Polyethylene Terephthalate and they should say Polyethylene Terephthalate somewhere on the package.
SAFLIP is a brand of flips that are "Mylar" or Polyethylene Terephthalate.