On some npc priests in Sto Lat.
Due to the cost of purchasing these, alternative methods of acquisition are often sought. One way is to employ a licensed thief to steal one from one of the many temple shops in Ankh-Morpork. Although the price will be greatly reduced this way, one third of the full price will have to be paid towards the thief's quota. Holy relics are also often for sale at Gufnork's temple and Sek's temple in Ohulan-Cutash. Any thief worth their salt (and many other people) will be able to pick one of these up at no cost.
For those without the requisite contacts or covert skills, a relic can be found on Rudolpho of Sek in the Temple of Small Gods. Dispatching him is no easy task, since he has seemingly boundless energy and employs both Creeping Doom and Fear against any attacker. Happily though, all priests are immune from Creeping Doom, and a couple of priests working together will, with a bit of patience, be able to take him down.