Etymology fun fact of the day: "shaman" is actually a term for a practitioner of Siberian/Ural/Altaic folk religion, and has nothing whatsoever to do with Native Americans. So when people write about Native American "shamans", their ignorance and lack of reseearch is hanging out there for all to see.
Or you could, you know, make up your own thing. What's the point of ganking material from another culture if you're going to change it so much that they don't recognize it/are offended by you trying to connect it to them? Oh, yeah, to make your story "exotic". That's what people object to in terms of cultural appropriation.
For example, look at belly-dancing in the West. Think about the stereotypes we have of the practice and its supposed "history". It's a great example of the sexualization of other cultures for our own titillation, which is one form of cultural appropriation. I'll bet you don't know that Western "belly-dance" is actually the amalgamation of not only multiple Old World traditions of folk dance, but also the introduction of jazz and dance hall traditions from the West. It has a long history and many connotations for the people in places where the various traditions originated. But the cultural imperialism of the Victorian British in Egypt conflates it with things like stripping and so-called "exotic dancing". (The meaning of that term out to be a pretty big hint to you.)
Now, the co-opting of Celtic mythology may not create the same sense of violation in your mind, but it's a pretty accurate parallel.
In creating your story mythology, be respectful of the cultures who you are using as inspiration: do your research and if you find yourself straying to far from what you've learned, then maybe it's time to drop the "Celtic" or "Wiccan" or "Arabic" label, and just go with the generic. Lots of cultures have myths of fairies and the Fair Folk, so it's fine to use those terms, but don't use the specific labels if the thing you're creating doesn't really match the truth of those labels.
