Finding a female Eevee is simply difficult since there is a 12.5% chance a encountered Eevee to be female. But you have to keep looking, but don't expect to find a shiny one since the chance of finding one is 1 in 8192. Also, there is no guarantee a female Eevee to be shiny.
EDIT: It may seem trivial, but in Generation 2, female Eevee`s couldn't be shiny at all! From Bulbapedia:
Due to gender being determined by IVs in Generation II, it is impossible to have a Shiny Pokémon with a gender ratio of seven males to one female that is female in those games, as the highest Attack IV a female Pokémon with a gender ratio of seven males to one female can have is 1, while the lowest Attack IV a Shiny Pokémon can have is 2.
