As Laurence has said, buying common seeds is a good option. It's a lot cheaper to buy common seeds from the Supplies Shop. You know what Reptans seeds look like, so keep buying them, and you will eventually get a Spotted Reptans. There are only 5 flower types possible on a common seed, so each time you have a 20% chance of getting a Spotted.

Mutation is much costlier, is not guaranteed, but it is faster than waiting for a new seed supply in the store. There is a chance that you will get a Spotted Reptans from a Painted Reptans. But you could also see the foliage change into a different plant altogether or get a Daisy Reptans.

If you don't have a Painted Reptans, you could try "working your way down" with mutation liquid, but it will be expensive and potentially very frustrating, since the plant could change in ways that are opposite to what you're trying to accomplish.

If everything mutated exactly as you wanted each time (only the flowers changed and only toward Spotted), you could work your way down from a Nox Reptans, as follows:

Nox Reptans => Daisy Reptans => Painted Reptans => Spotted Reptans

Good luck!