Question about duvet inserts

So I was recently gifted an anthropology duvet cover. I don’t have an insert and checked the website and oh my god those are pricey! I was wondering can I just get a cheaper insert from target or something? Will that fit? Or do I have to get the specific insert off the website?

Yes, I know this is probably a stupid question, but I’ve never had a duvet before. I didn’t even know what one was until I had one gifted to me recently so discovering they need inserts is new.