Are there free hostels/shelters/accommodations in Poland?
No free hostels, I am afraid. There are free shelters for homeless people. As for free accommodation of another sort, try squatter groups.
For example:
warszawa.gazeta.pl/warszawa/1,34889,11412417,Squattersi_biora_sprawy_w_swoje_rece__Jest_nowy_squat.html
forum.gazeta.pl/forum/w,752,134533378,,Squattersi_otworzyli_kolejny_squat_w_Srodmiesciu.html?v=2