The ones I can think of being:

1) A Jedi Temple.
While the rules on Force ghosts aren't quite clear (in either canon, so far as I know) it might be possible for your character to stumble across some lesser known Jedi outpost and be confronted by a guardian or spirit there, and be promoted by them.

2) Someone who knew Jedi tradition well enough to decide if your character would be worthy.
Perhaps a historian of some description, a member of another Force using group (depending on whether this is Disney or Legends), a notable associate of a Jedi Master, or someone who worked in the Temple itself. I know you said it would be selfish of him to Knight himself, but perhaps it would be different if he didn't make that judgment himself? I dunno.

3) A Jedi Holocron.
While the Holocron gatekeeper (provided those are still a thing in Disney canon) wouldn't have the authority to actually Knight anyone, it would certainly know better than most when a person is worthy of the title, and after a while of being considered a Knight in all but name, why not just accept it? Goes back to the Knighting himself thing, but I'd say it would be entirely reasonable to consider yourself a Knight if the closest thing to a Jedi Master you're likely to find says you've earned it. Suppose it depends on the character, though. And there's the rather significant problem of how to find a Holocron.

4) A former Jedi.
While Jedi Masters leaving the Order wasn't a common thing, there are enough instances in Legends to justify making one of your own, and if it's Legends, then there's a whole new canon to work with. Of course, that also means there's a whole new canon to trip over, but I'm sure you could make it work. As long as you could find a plausible reason for the Master to leave the Order (the horrors of the Clone Wars, for instance) then I see no reason why they couldn't survive Order 66 long enough to meet and promote your character. Hard to track a Jedi who's not a Jedi anymore, after all.

Hope that's a least a little bit helpful, and if you need any more help feel free to drop me a line. I may not be quite the expert I used to be, but Star Wars remains my all time favorite fandom.