Hi Josie,

You can buy the Cuzco tourist ticket from any of the sites at which it allows entry. There is also an office on Avenida del Sol which sells them, in the basement of the tourist office.
There are quite a few different options, depending upon what you would like to see, and if you want to see the majority of sites included in your ticket, then this will definately be the most economical way to go about it. However, have a good look at which sites you want to visit, as it does not include quite a few of the bigger ones and therefore you may find yourself with a ticket to alot of sites that you don't really want to see and only a few that you do. In short, don't assume that it includes entry to the places that you want to go.
It's not cheap, the full ticket which includes Pisaq, Saqsaywaman (I'll never get that spelling right!) and Ollantaytambo in the sacred valley as well as Qoricancha (can't spell that one either!) is around USD55-60, however once you add up the entrance fees at each place individually then it works out cheaper.

Hope that helps!