You can use your imagination to practice the straights and turns you need for barrel racing but there are certainly lots of drills and exercises out there. In fact, there is a site that has a pile of them (Barrel Racing Drills for Barrel Racing Success)
One of my favorites is circle tires where you put 5 o 6 tires (I use bicycle tires) in a circle about 50 -60 ft across. You circle the big circle until your horse is attentive and soft then you would position yourself to circle one of the tires. You want to do a perfect circle around it (this practices maneuverability and control, softness and bend). You can turn every second or third one to start with, however many you need to skip to approach the next one good. You can start at a trot, but eventually you want to be able to lope and lope a perfect circle around each one. This is good for young and old horses. It builds the muscles your horse needs to turn and puts the moves in them too.