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The reason I am asking is because I can not seem to find unmodified potato starch here, so I would need to order from overseas.
There should be no need to go overseas. Just yesterday I bought an eight pound bag at the grocery down the street for only $2.69.
Seriously, see fairy's posts above. Packaged extracted potato starch is convenient for adding to other foods, but the most unmodified stuff you can get is in the produce section. Bake one and season with some coconut oil and you have a tasty and nutritious treat.
Potatoes aren't that bad raw, either. Aside from water, they're mostly unmodified potato starch with a bonus of a little protein, fiber, vitamins and minerals like potassium, and sundry other nutrients. Although I don't have the source handy, i recently read that cooking potatoes and then cooling them overnight also produces resistant starch. Potato salad anyone?
In general, it's clear that if you want to seed your digestive tract with specific species of bacteria, probiotic foods and commercial supplements are the about only game in town. But I'm not convinced you have to resort to esoteric commercial products to feed them well. The same kind of diet that is healthy anyway, one including a plentiful variety of plant foods - unmodified to the extent palatable - will also help keep your gut bugs happy.