@xenioPL You have a couple options. I used an acrylic from www.tapplastics.com in my mirror. It was much cheaper than an equivalent glass mirror.

I have a roll of the one-way mirror film, which is probably your cheapest option. It just requires more care when you apply it than a glass or acrylic piece. On the positive side, you can use regular glass with the film, which makes sourcing easier.

I bought the one-way film from Amazon.