The females tend to be less colorful and harder to sell. If the stores sell only males they end up stuck with a tank of females. So they require pair/trio sales. However, if you look in the feeder tanks you can usually find a number of female guppies. These have not been well cared for so expect to quarantine and nurse back to health with about 50% die off. The surviving females will do well with your quality males in time. They must have several birth episodes to get rid of sperm from other mates. OTOH, you may get some spectacular young from them and the purchase, even at a 50% die off, will be much less costly. The females may not be fancy guppies but will usually input strong genetics into any breeding program.
Charlie