Baking fish and I don't have parchment paper. Can I substitute with another paper or tool?
You could bake on foil but it can stick so I would lightly oil the foil.
Silplat is my go to. However in a pinch at low heat (below 400) you can use brown paper bag cut open, and depending on the application rub it with some flour.
Hint for storing Silplat: Rull it up and put it in the tube from a roll of paper towels. Silplat is well worth the up front cost IMHO.
Be careful with paper bags! Many of the grocery bags have inks that may become toxic when heated. There are 2 new parchment paper/foil in one that are great. "MarhaWrap" by Martha Stewart is great...you get the "creasing fold-ability" of foil with the non stick of parchment. But I do agree with Sam that Silpat is worth the investment.
If you can find it, liquid lecithin is an amazing pan-greaser. Nothing sticks, and if you rub it in with your fingers, rub what's left onto your hands like lotion -- amazing, too.