In my opinion there is no separate sampling method for endangered and endemic species as such. Because the IUCN classification of endangered and endemic grouping is on population only but geographical area or extent will vary. So it is equally important that which is study area and in what reference the species is endangered/ endemic.
I do not think there is seperate sampling method for endangered species. And even if there is, geographical variations interms of species distribution matters alot. You must understand the area you studying first, it's ecology before you begin to think of how you can sampling what. Find Kent and Coker ( 1992) for further information. Good luck