I suspect that if this is a high end or pro pastry magazine (or British or Aussie) they are referring to an acrylic sheet. We also use acrylic strips in chocolate work. A chocolatier/high end pastry chef would readily have such a product available in the kitchen.
Probably waxed paper. Parchment and grease-proof (i.e. coated butcher's wrap) are different products.
You can see through waxed paper and it releases relatively easily from set buttercream and royal icing. HTH 
The wax paper is available at any grocery store the brand name is Reynolds Cut-Rite wax paper next to the aluminum foil in the paper isle (blue box). I would use this for royal as it will be easier to release once dry. Allow to dry for a few days the turn the plaque over and gently release/roll the wax paper off slowly away from the plaque, good luck!!
The plasticy sheet protectors that you put pages in so you don't need to punch holes will work. It needs to be flexible so that you can peel it off the run out once it is dry. Polish your pocket first and peel it off with tension ( like the mat video shows to separate mat and fondant). HTH
Yours aye
Puss
OP said: "want to make a royal icing plague "............
Yes, the plastic page sheet will work however it is not food safe.
Plastic wrap (in the US) is a much better product to use for this technique.
Put your pattern on something solid - like a cookie sheet - and cover with plastic wrap - making sure it is wrinkle free and go to it
The finished, dried item will come off the plastic wrap easily. If it doesn't it most likely is not dry completely through.
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