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.

It's really not that much of a time commitment. Once you get it going, there are ways to store it (fridge, freezer, dehydrated) so that you can keep it long-term, with limited upkeep. Of course, it will always need care and feeding to get it active again. Short of that, the solution to your problem is to find a friend with an active starter. It is easy to transfer an active starter to someone else for their use.

There are prepackaged portions of sour dough starter available in most grocery stores. They have a shelf life of a few months and don't need to be fed if not opened. This is just one of a couple variants that are available in my next grocery store.

As with every sour dough starter you can just feed it to grow the colony, then use half of it and keep the other half for later. It will last for at least a week without feeding when stored in the fridge, multiple weeks are also possible but it will probably need some time to recover.

You can make bread that has some of the properties of sourdough without the full "starter" treatment. It won't be the same, of course, but it might fit the criteria you're looking for.

One option is to use sour ingredients, like greek yogurt, such as in this recipe.

Another option is to use a poolish, which is sort of like a sourdough starter that you make a night or so before, but don't keep beyond this recipe. You usually make about half or so of your dough, ferment it for a day or so, and then make the rest. Here's one recipe for a classic French boule that uses the poolish method. Again, this isn't sourdough bread, but it has some of the same characteristics, and you don't need to do anything to maintain the starter - though it does take extra time, of course, since it has to sit for quite a while (but at least it's not active time).

My routine is to make sourdough every few days, keeping the starter in the fridge and proofing overnight in the fridge. But sometimes I mess up my routine and need a sourdough loaf same day. That's when I cheat, using yogurt whey and rapid rise yeast. Most people think the loaf is sourdough.

I make Greek Yogurt several times a week and refrigerate the whey. I've kept it for weeks and never had it go "off". If you make a regular loaf substituting yogurt whey for the water, the bread has a nice acid "twang" to it.