Most saddles come with a spot to put a back cinch on, but there are many saddles that are sold new without one...even if they have a place for it.
IMHO, whether you need one or not greatly depends on how your saddle fits your horse. Does the back pop up when you rate coming into the barrel? Does it ever feel like it's moving around too much? If not, then I'd say you don't need one.
However, if you answered yes to either of those questions, then I'd suggest you get one. You wouldn't need a heavy duty expensive one. Something like this would work fine Fabtron Double-Ply Leather Flank Cinch - Horse.com
Of course, you can find full flank sets that have the billets included, but they tend to be more expensive than getting the cinch and billets separately...but matching can be hard. Circle Y Floral Flank Cinch with Billets - Horse.com
Personally, I won't go any narrower than 3 inches on a flank cinch for regular riding. I don't want there to be too much pressure if it does ever come tight.