Players afaik.
The op is right, asides incredible patience, or luck, the likelyhood of seeing them is going to be hard.
Kinda needs an extra mechanic, like going to a place , and paying an 'informer' to alert you as soon as they show up there... managing a network of your informants then might be key
At the risk of breaking the 4th wall, maybe you'd even get the alert by email even outside the game. Even if you aren't around, , at least you might start to see a pattern... of course finding someone with a bounty and claiming it are rather two different things!