Jackson, Manbushijie are really just offering ready made homage from a supplier. U order from them, they order from supplier and ship to u.
Getat has both. On his website, those that have the word custom are put together by him. Those that don't are ordered from a supplier but he gives you some options like choices of straps, etc. He also does custom jobs and is quite reasonable. Look thru his parts list and email him to start discussion on what you want. You can choose the case, dial, hands, movement, etc. Only offers asian movement.
Davidsen is about 4x more expensive than Tat but from my experience is by far the best among the 3. From the h forum you can see that even Panerai owners order his customs. Offers Swiss movement.
No experience with RXW yet. Still looking for info on how to get one.