Hi Elseven,
There is some general information about sheets and bedding in post #7 here and the posts it links to that may be helpful.
While you can order twin XL fitted sheets and a king top sheet and king pillow cases separately … a split king sheet set is more difficult to find. There are a couple of sources for them in post #26 here (one of which is the hardtofindsheets you already mentioned).
Hopefully some of the forum members that have purchased split king sheets will see your post and share their comments and feedback about what they are using as well.
Phoenix