Anyone know where I can get an SDB other than from the Registry that the yacht is registered with?
Why don’t you want one from the flag state?
If you stay on yachts you will probably spend most of your time on Red Ensign Group boats so get one from the MCA.
I represent a group of Nepalese who serve on a vessel where the flag registry only issue the books to nationals of the flag. They constantly have issues when returning to their home country, and in gaining relevant visas.