Hello Wolly and welcome to the forum. Piwo's recommendations are sound. As far as good sites for solid information and study of genuine Polish militaria, you've found the best English language site on the web right here. There's a lot of stuff packed onto the discussion threads, so be sure to pore through them. If you find something in the marketplace that interests you it's always a good step to post the item here for comments. As Piwo mentioned, there's plenty of fakes and copies floating about so you need to be careful before buying.
Regards and best wishes with your collecting,