Princess bees are necessary for mating queen bees.  A Princess can be found in Hives in almost any biome.  Breaking a hive with a scoop will always produce 1 Princess, as well as one or more Drones.  Also, once a Queen has finished production in an Apiary or Alveary, she will create a Princess and at least one Drone.

Princess bees can found or created in any bee species and are crucial to bee production.  Outside of any genetic manipulation, you will never have more Princesses than you find in the wild.  This is because every Princess you breed will turn into one Queen, and every Queen will make only one Princess (using up the Queen).  The only variation being the breed and characteristics, which can change from generation to generation. 

See Bee Breeding for detailed information.