Comfrey is abundant in Europe, most notably Britain and Ireland. It grows in damp and grassy places, usually along river banks or ditches that can fill with water. Since it's a perennial herb, it can also frequently be found growing in gardens. Many intentionally plant Comfrey, dressing the bed with manure and keeping it well-watered.
Stinging nettle is a perennial herb that is abundant throughout northern Europe, north Africa, and a majority of both Asia and North America. In the United States, it can be found within every province and state except for Hawaii. It grows commonly in the countryside and among abandoned buildings.
Yarrow can grow anywhere, even in poor soil conditions, but does best in a sunny position with good drainage and light soil. It can be cultivated in gardens, and is primarily native to the Northern Hemisphere.
A word of caution, though: While it's nice to know that the plants used by the medicine cats in the Warriors world are legitimate medical herbs in our real world, you should not try to find them or use them on your own. There can be side effects to each.