That's a very early period and examples would be limited in the sense that you won't find too many in Polish museums or even recorded in paintings of the time as the dominant theme in art was religious imagery. I suggest you read the Wikipedia article on warfare in medieval Poland. There's quite a lot of detail in it about armour and weapons. Some of the armour worn was universal in that it followed general designs of that period throughout Europe. Once you know which items were worn you can search for those items as opposed to just searching for Polish armour.
This site might be useful to you:
And this one:
Lots of images here, mixed Slavic warriors so you have to keep scrolling but there are Polish examples: