Oh, for.... it is so easy! Look here:
http://guppy-guide.blogspot.com/2010/01/sexing-guppies.html
Good picture of the differences on the above site, my only correction is that the site puts the label gonopodium on both the male and female, just saying the shape is different. Correctly, both the male and female guppy have an a*n*a*l fin, but in the male the a*n*a*l fin is modified into a tube-like gonopodium (for impregnating female guppies) whereas the female's a*n*a*l fin remains a normal triangle shape. Otherwise the site correctly show the differences in the sexes.
I worked in the aquarium business for over a decade as a teen and putting myself through college. I will say that some of the big pet stores and definitely the box stores like Walmart may only carry males, because most frivolous fish buyers are only interested in "pretty" not breeding. And the stores don't want to be left with a bunch of un-bought dull females.
Edit because this site's family filter always edits that word! LOL