Purchasing a SIM card in Mostar?

Where can this be done for a good price? My friend and I had purchased international SIM cards only for the balance to get completely sucked dry as soon as we crossed the border. We like to have at least data so that we can use whatsapp. Also if anyone would like to show us around the way a local sees Mostar we would love to meet up and buy you a beer! Cheers!