Now the concert is sadly over and you want something physical and not pictures to remember it. That is what merch is for. This page will let you know what the prices are for items, what may sell out, and when to get the merch.
When you first get inside the venue, go to the merch booth to check out what's for sale. The line could be short so right when the doors to the concert open, now is the time to buy merch. If the concert you went to has a special shirt for the tour, that may sell out in certain sizes so the earlier you get the shirt the less of a chance it's sold out.
Clink the link to see pictures of concerts I've been to.Pictures I've taken at cocerts.