Most good leather or upholstery shops can replace sections of leather, but if the rest of the seat is well-worn, it won't look right.
I bought all new seat leather from LeatherSeats.com and installed it myself. The leather was top-notch, and I could choose the colors and patterns I wanted. I also sent them my foam cushions for re-conditioning and repair, but that may not be economically feasible for you. They have several Youtube videos on their products and processes. In fact, my Z3 sport seats are featured on one of their videos.
I don't know if they ship overseas, but if they do, you can find a local shop to replace the seat covers or do it yourself.
If you have standard (non-sport) Z-3 seats, I have a set of new black leather covers I will sell you. I bought them before I realized that I had sport seats.