What is the source for slaves being sent to the Land of the Dead? Criminals. Raids on the Koryni (one of the sources of the cold war between the two are these raids by "unknown agents" according to the Holy Cities). Old and crippled military veterans who are making their final journey. Old and crippled slaves in general. Prisoners of war against other cities/peoples in Al Allahan. Greatest prices are for children and those of child-bearing age.
Slaving ships to LOD are not that common. Mostly go to Bonehold and Castle Tyrantus. Prices paid in ancient coin, artifacts and the remaining vast wealth of what the buyer's call the fallen Korthrim Imperium.
Slaving operations run by criminal gangs, licensed slavers and a sect of a death cult? City of Nergal, god of the dead, is known as the greatest market for slaves.