If your concert's event is NOT listed on TicketMaster, check your respective venue's site. Once you get there, check out other concerts in which the tickets are already on sale. It will show you where to buy them. Both Houston and Denver's arena's (Toyota and Pepsi Centers) both use a system called Ticketing Central. Which means you have to actually buy the tickets through their site.
I emailed the Toyota Center earlier asking about the info and all that. Waiting for a response. I would assume that whatever they tell me would also apply for any other places using ticketing central (you will know when you click to buy tickets for another even such as Cher) as it will have some form of ticketingcentral.com URL.