One plant can be found in the fenced backyard (southern most side) of the Goodsprings house where Sunny Smiles and Trudy head to for the night. Although it is marked red, no Karma is lost when the fruits are picked up.
On the shore of Lake Mead, also on the shore near the crashed B-29 Bomber.
At least 2 prickly pear plants can be found just south of the Northern Passage, near the shack where the Jackal gang members have set up camp just outside of New Vegas; as you walk down from the Northern Passage, you will see smoke from their campfire; that's the best way to find the location easily. Two Jackal Gang Members reside there; get them out of the way, and the prickly pear is free for the taking. Near the same location, you will also find at least 4 barrel cactus fruit, two Nevada agave fruit, one honey mesquite plant, and one banana yucca fruit.
3 plants can be located just east of Mojave home, where the mad brahmin reside; two of the plants are near the big rocks in the back.
Carrying prickly pear fruit will give you four times more hydration per weight than water. This bonus rises to 7.5 times (1285 H2O recovery per pound of weight carried) with 100 Survival and the Pack Rat and Camel of the Mojave perks. Without Camel of the Mojave (because of consuming prickly pear instead of water) this rises further to 8.6 times (1285/150).
Turning two prickly pear fruits into one bottle of purified water increases the H2O benefit from 90 to 150 at 100 Survival at the expense of 0.86 pounds of weight over simply carrying the fruits by themselves.