Yasir appears completely randomly, it can take a month or more before he appears and sometimes he appears several days in a row.When Yasir spawns it is announced on the The World Board in the adventurer's guild. You reach the adventurer's guild by using an adventurer's stone in any temple in tibia. You get the adventurer's stone by asking for it from the temple npc.Ellotris Guardian commented that "NPC Towncryer in Thais announces Yasir's arrival as well (but he does not specify the town Yasir is in).".
When he appears in tibia he can be found in one out of three places.
You can find map locations on the top of tibia wiki's Yasir page
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