Best option according to advice for previous trip is to take English notes and go to local 'Kantor' (I think?) booth.
We did this underneath Warsaw Centralna station and got a much better rate than available in UK and also better
than mate who went to bank in Warsaw.
We got more than 500 Zloty for £100 with no charges which was v good at the time.
Apparently, the booths all vary, but I just asked and made sure there were no charges.......no problem!