Where can one see a slow loris in Phuket, legally, while avoiding scams that seem to be associated with this protected primate?
Simplest- the zoo
Plain link attribution: https://zooinstitutes.com/animals/northern-slow-loris-phuket-zoo-14525.html
I would like to add that I don't recommend Phuket Zoo. There are better zoos out there (in the 'area', Chiang Mai Zoo, Khon Kaen Zoo and Singapore Zoo come to mind as better alternatives, though you'd have to check that they have this animal).
However, if you must see it and you can't travel off the island, then I guess the zoo is better (in terms of animal welfare, etc.) than some guy on the beach (which based on the linked article is probably illegal too).
A bit more effort, perhaps more rewarding
Another option would be to contact the organisations that you would contact if you found someone in possession of this animal illegally. Given how many tourists want to see them and how the tourist industry plays into that, you might ask them where you might find one that's been taken care of properly, perhaps one that is still recovering from being released into the wild. I found this article by redeyethai.com, which links to a Facebook page for a rescue organisation on the island.
A good option might be to contact them via Facebook to inquire about the best option (i.e. not helping those profiting from animal cruelty but instead helping those who try to prevent that).