Most probiotics aren't useful because if you're going to manufacture a ton of bacteria, you have to go with the ones capable of growing that fast. That's typically just one or two strains used in the making of yogurt, though you could branch out to miso, kefir, kimchi etc.

And the oral delivery route doesn't work that well because they have to make it through the stomach, then get to the intestines, then survive there - which they won't if they aren't specific strains evolved to live inside humans.

If you want to do science (aka mail someone your poop) there are ways to measure what's currently inside you:

https://www.viome.com