Thirty seconds and one google search later, here are some epub to mobi converters.
https://convertio.co/ebook-converter/ (They don't have a way to convert a word file to an epub, so you need to at least have enough technical know-how to create a mobi to begin with, but this is easy enough to do. Just change the .zip file extension to .epub and you will get the same files then convert it to mobi or azw.)
https://www.zamzar.com/ (Has a much larger library of documents they can convert, including word docs, but there is no promise that the azw or mobi is the same file as the one Amazon uses, for this reason, I would take the HTML files, change the file extension to epub and then convert the epub to the amazon file.)
https://calibre-ebook.com/download (This one is the one everyone recommends, but you have to know that it will change the css. On the surface, this isn't a problem, it basically takes the styles that are the same and converts them into a calibre named style. This is totally problematic if you actually use specific names in your css so that you can use the same css across all books and just make a few tweaks as necessary (I use a default CSS then open it up inside Sigil then make necessary changes for series branding, then just use that css for all the books in the series, calibre makes it difficult to do that with the renaming of styles.) I know people swear by Calibre for creating files, but I think Sigil is a better creator option and using Calibre for conversions.)
https://ebook.online-convert.com/ (No frills converter, but haws a massive amount of files they can convert.)
https://epubee-converter.software.informer.com/ (Hasn't been updated in a long time, but this one comes well-recommended because the publisher/creator also has a DRM remover that is one of the best out there. They also have a maker (converting epubs from word files).)