Hi Alexandr,
I'm building this desk at the moment. I'm using plywood (18mm) of "CD" quality (this is almost the lowest quality available). I was lucky to find a nice batch at my local hardware store (as they are $60 AUD per 2400x1200 sheet) where the finished surfaces were good enough.
Alternatives would be marine ply (grade "AA") which is usually available at local hardware store but here in Australia it is ~$130 per sheet. Or a specialist plywood supplier that can offer all sorts of nice finishes and grades but usually cost little more
Hope this helps
Greg