Hmmm.... Tricky one.
If you're meaning someone like Huggy Bear from Starsky And Hutch, or Jonny the Shoe Shine Guy from Police Squad, then possibly. I did include "consultants" in the list of playable character types.
However I'm not sure whether US police departments still make use of paid regular informers.
If you're meaning someone who just happens to know who the murderer is, and wants paying before they offer the information, then no, that would be a NPC.
However, here's the thing: In the 1970s, every cop show had the "source" character. Today, none of them do. And I think the reason is that modern police work relies a lot more on forensics and less on the rumour mill. That said, I've had a think, and I actually quite like the idea.
Here's my thoughts on the sort of thing such a character would do....
They would probably be an ageing hooker or pimp, and would be asked by the detective to see what they can find out. They would then need to ask around, meet up with NPC characters, gossip, see what they could find out. Part of why I'm liking the idea is that such information is going to be uncorroborated at best and misleading at worst.
So, I like the idea, but I will only approve one such character - If RedEve decides she doesn't want it, then first come first served!