Every live show has a promoter. It's the promoters job to make awesome flyers and posters for the upcoming show. The poster get hung at record stores and different locations all over town. Also at the venue where the show is located the promoter will hang posters on the walls for the show. So when you go see a show look around on the walls or in the window for posters. Once the show is over there is no problem with taking down the poster and taking it home with you. You could even bring a pen and ask the band members to sign the poster. See a bunch of shows, grab a poster at each show, hang them on your wall and there you go, a punk rock room!
If you are too young to go to punk shows or live in the middle of nowhere you can buy punk show flyers here.