While the potions seem to be gated by logic skill, actually discovering them is random. If your sim has maxed out logic, then it's just a time sink. Still, IMO discovering the potion is easier and possibly faster than finding a rainbow gem.
In addition to the suggestions above...If you have World Adventures you can often get rainbow gems from exploring tombs, or just collecting gems in the WA worlds if your sim has the Collection Helper. Or, if the sim has completed the Timber Terminator skill challenge from the Martial Arts skill, then breaking space rocks can get you one. If the sim has completed the Body Builder skill challenge of the Athletic skill they won't fatigue out when breaking boards.
If the sim is old enough and time is an issue in the homeworld, I'd send the sim to university for a single class. Then have the sim spend most of the term at the chemistry station, or mining, or breaking boards, or gem collecting (although I don't recall if those normally spawn in the university world).