There are some, but the majority are male. They're an end result of the Schola Progenium programs, which take in the orphaned children of Imperial officers and raise them under a program of love for the Emperor and strict physical and mental training. The intellectually inclined tend to wind up in the Ecclesiarchy (should go without saying that, since the Ministorum runs the Schola Progenium, that they get first pick of the results).
The physically inclined are geared towards combat roles. The men are formed into Storm Trooper squads, and put through the most rigorous training the Imperium can devise. The end result is some of the finest soldier in the Imperium. The top 5% wind up serving the Inquisition as their private army, the Black Guard, and the remainder serve in the Imperial Guard.
The girls tend to wind up as part of the Adepta Sororitas, whether as the more famous Sisters of Battle, or one of the more mundane roles, such a Hospitaller or Famulous.
Commissars come from two sources. One: decommissioned Storm Troopers. One of the elements that make them so effective is the lifetime of training and service with the same squad, allowing a bond and level of communication that isn't possible otherwise. As such, once a squad is under half strength, they're returned to the Schola and given further training to serve as commissars. And two: the child shows an unusually high level of zealotry and devotion to the Emperor, marking him/her for Imperial service. Again, this would apply predominantly to boys, as most girls who showed such zeal would be sent off to an Order Militant.
Long winded way to put it, but the basic gist of it is yes, there are female commissars, but they are much, much rarer then their male counterparts, since the criteria that gets a boy earmarked for commissar training is the same as that which gets a girl earmarked for the Sisters of Battle.
That really helps me out actually, commissars is one of my favorite positions within the imperium but I've never understood much about their training or selection, I just assumed they were adeptus arbites drop outs, but that clears up a lot and helps me as an artist since I am intent on drawing a piece involving the trigger happy disciplinarians. Again, Thank you.