Looking For Cheaper Tickets

Mama Mia is performing in here in our place for two weeks. All the performers came here all the way from London. I would love to watch. Some of my friends are going to see the show on Sunday and sad to say I can't come because I am working that day. I need to look for another schedule that can fit in with my two girl friends. At the same time I am looking for lady gaga concert tickets because my two girls would like to go to one of her concert. Do you knoy any site that you can recommend for cheaper tickets?