see the Stavanger folk club website
also there's an Irish pub in the centre which I understand hosts sessions and there was an old thread here about the a ceilidh at the Stavanger Rugby Club (you say you looked up the old threads so you probably saw that*) ... the Norwegian traditional music can be harder to find if you don't have the contacts, though there are some good Norwegian folk festivals with emphasis on local music.

*relevant old mudcat threads include
Stavanger A
Stavanger B
Stavanger C