Since I have oodles of polishes, I really don't have a problem finding a few colors in the same group to use for a gradation look. But if you don't own hundreds of polishes, or just want to make sure that the colors you wear go together, check out these fab Kiss Gradation Polishes. They take all the guess work out of color-picking and make the ombré look easier to achieve.
Each set comes with three coordinating colors (personally, I'm a fan of the pink set, though all of them are super pretty!) and instructions on how to create the gorgey look in three easy steps!
My only concern is that the set does not come with a makeup sponge. It's my preferred tool for maximum blending. But after checking out a few swatches online, it looks like the colors are sheer enough to blend on their own. Of course you can use a sponge if you feel that it's necessary, though. And for under $7, it's def worth a try!