pawel Hi there and welcome! If you are looking for an actual 3dfx Voodoo 5 6000 (any version), it's pretty difficult as of late. Either way you will need some time and money for this journey. I managed to acquire one last year, but it was not easy nor cheap. Because there not many of them to begin with, it's hard to find. This being said, you are certainly at the right place and everyone is super helpful here.
Also, I don't know much about it but I was made aware of a 3dfx voodoo 5 6000 replica card that someone designed and built which is like the real card, just better in ways. I think it has up to 256 MB vram, is not as AGP/voltage picky and has a good internal power source connector and such. Maybe someone like Obi Wan Kenobi can chime in on this. I think he knows much more about it.
Just for starters.