I have seem some references to Harald Fairhair supposedly banning prostitution and establishing short term thralldom as a punishment. But I cannot find any references to it beyond people mentioning it on reddit and some random internet posts. Supposedly Tormod Torfæus' Historia Rerum Norvegicarum mentions it but I cannot find a buy able copy or even a excerpt about the topic. I would appreciate any help here.
First of all, put it simply, neither do I have access to copies of modern Norwegian translation of Torfæus nor Latin original right now.
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Apparently their ultimate online source (at least in English) would be Marit Synnøve Vea, "Viking Women," hosted on Avaldnes Historical Museum.
.....Well, are you really looking for Torfæus' description as a source to the policy of "Harald Fairhair" in real history?
It would be a bit difficult to define Tormod Torfæus (1636-1719) and his work purely from a modern, source criticism point of view. In the 21th century, several researchers have reconsidered characters of his works. He was about a contemporary person with another famous Icelandic antiquarian, Árni Magnússon (1663-1730), and lived before the establishment of philology as well as history as modern disciplines. It is true that his extensive investigation of medieval Icelandic and Norwegian manuscripts might enabled him to include some of now lost materials, but it does matter little on the alleged reign of Harald Fairhair (d. 932/33?), since we don't have much contemporary source on him except for a few skaldic poems, as I summarized before in: Did Harald Fine/Fairhair actually exist? and/or Was Hakon the Good a real person? More generally, how do we know people in the past really existed?.
I'd especially recommend the article of Roggen and Jørgensen (translator of Historia r. Norvecigarum), or at least the online entry of Norsk Biografisk Leksikon before treating it as a possible valid source on Harald Fairhair or the Viking Age Society.
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(Added): I also forgot to mention the possible but important problem on the legal concept of "fredløshet" (ON friðlauss) mentioned in the linked article above (for its application to the early 10th century Norway).
There are in fact various terms to denote the basic concept of outlawry in Old Norse (Cf. Riisøy 2014), and their provenance (together with the validity of largely later law texts to reconstruct Viking Age Scandinavian society) has heavily been disputed especially since 1970s without reaching an academic consensus. It is true that Riisøy recently claims that some of them like skógr, vargr and sekr could actually date back to pre-Christian Norway, but even if we accept her hypothesis, "fredløshet" (ON friðlauss) and its accompanying concept, "fredkøb", can be problematic.
While Torfæus' text is written in Latin (so it might be not easy to translate the concept into Old Norse in somecase ) - it's a problem per se, the text apparently also states that the penalty against the prostitution can be redeemed by the payment of fine to the king. This is the basic characteristic of "fredkøb", but we are not sure whether the king really had much voice in (corrects): enforcing the law on such an issue (giving a pardon against the crime....but against what kind of crime by committing prostitution?) in pre-Christian Scandinavia. The classic study on medieval Scandinavian Law like Ole Fenger regard this characteristic of "fredkøb" primarily as a later, Christian Influence (Fenger 1983: 74-76).
It should also perhaps be emphasized that even later Saga traditions specify not Harald, but his son, Håkon the Good, as the first legendary lawmaker in the 10th century Norway (that would rob Harald of legitimacy of stipulating such a ban further).
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