Legacy.com obituaries usually have the original source at the bottom of the page – hardly any of them are unique to legacy.com. If it originally came from a newspaper, I might cite it like this:
"John Lastname", Newspaper Name; City, State; Publication date, via legacy.com (Link to legacy obituary, accessed date).
or if it came from a funeral home, I try to check the funeral home's website to see if it is still available there.