I wanted to use a summon demon ritual in my campaign and under the component cost it says plus a focus worth of 5,000gp and I am not too sure what the focus is or how it is obtained. Any help would be greatly appreciated.
The focus is a circle of summoning runes. It and its associated ritual are described in Demonomicon, page 18. The general rule for focuses ("focii" for the nerds) are in the PHB chapter 10, see especially page 300.