If circlips were provided with a piston set, I would think it best to use those.
If one must shop locally for circlips, I would try an industrial supply house, rather than a hardware store. Industrial suppliers will have better quality, and ought to have ground circlips which have a sharp edge on both sides. Stamped circlips have a sharp edge on one side only, and as htown16 says should go towards the outside.
Slick