One time sourdough starter
I would recommend you find a (hobby) baker locally. Almost everyone who maintains a sourdough has some extra that they would otherwise discard. I have read about local Facebook groups etc. of people sharing their starters during the pandemic (when sourdough suddenly was “a thing” and weirdly enough yeast was hard to get at times), including  contact-less drop-offs and similar. I have shared my starter(s) freely with everyone who asked for it in the past. Even repeatedly for one-time-bakers as you are planning to be.
The charm of this approach is that you can make new acquaintances that way and that the shared starter most likely will come with a bit of an explanation as to the specific culture’s temperament. They might even be willing to share a few favorite recipes and give you some hits that a generic recipe doesn’t have.
That said, if you have an established starter, it’s really not that much of a problem if you bake only once in a blue moon. Mine is parked in the fridge, sometimes for months on end, without further care. A bit of pampering and it’s as good as new, ready to climb out of the jar.