Alternatives to acetate?
Rarely. Acetates are usually used when you need a thin, flexible backing for whatever you're making. Silpats aren't nearly as flexible, and don't have a uniformly smooth surface; you will impart a texture to whatever you're making. And you can't really cut them into whatever shape you want. Parchment paper doesn't have the strength of acetate; you can't make rings out of it. Plus it will absorb some liquid, lowering its strength further.