Your best bet is to use a commercially available sour blend such as Wyeast lambic blend, Wyeast Roselare, or White Labs sour mix.
They all have saccharomyces in there to get your fermentation under way plus a nice blend of brettanomyces and bacteria. Unfortunately these blends are not very robust and the many times the finished product will be underwhelming in terms of acidity and sourness. If you want more sourness (and who wouldn't
) then I recommend pitching a couple bottles worth of dregs from your favorite sour beers (lambics, gueuze, Jolly Pumpkin, etc). This will add to the complexity and boost the acidity as well.