If you have a design or motif in mind, for example a type of tree or a repeated pattern of stars or plaids you want to re-create on nails, you can bypass the premade nail art and salons and make your own stencils. Craft stores are great sources for adhesive paper such as laminate or sticker sheets, and scrapbooking sections have hole punches and scissors in all shapes and sizes for making unique cutouts.
Something as simple as scotch tape will work too, although it can be too thin for a complex design. Masking tape or other, tougher types of tape can be cut and adhered tightly to keep nail polish from seeping underneath, and stay strong enough to come off in one piece without tearing and messing up the paint job.
It may take some trial and error to get the design you want and the material that will work best, but it's fun experimenting and easy enough to try and try again with some nail polish remover and ideas.