By "fermented out" I assume you mean either you took a hydrometer reading and it was low, or you tasted it and it was not sweet?
Given how easy it is to try again, I would not wait for the souring bugs to suddenly start doing something. Or rather, I'd let it keep going, but try again at the same time. The following (and also Al's suggestion to keep the temp up) should increase your chances of getting whatever souring bugs are on the grain to take over:
1. Exclude oxygen from the process as much as possible. All the papers I have seen have found that oxygen has a really negative effect on growth in small-scale batches of souring bugs.
2. Excule other critters from the process as much as possible. (Obviously.)
3. Maybe hold the grain/wort mixture at, say, 150F for a few minutes to kill off everything BUT the thermophilic lactobacillus. That temp sounds familiar... in fact you could just take wort straight from the mash tun next time and not boil it. THAT sounds familiar too--from Berliner Weisse.
I have not done this. I have heard firsthand accounts of people doing it, with the results ranging from good to bad. Obviously you are taking your chances with what's on the grain. Killing off the wild yeast with high temps helps, but still some of those higher temp bacteria are good (l. delbrueckii) and some are not (for instance Boon's comments on certain "thermo-bacteria" in Wild Brews).
I *have* tried souring wort with the Wyeast lacto (which is not delbrueckii) at 86F and did not get much sourness. I think oxygen may have been the culprit but who knows. If I were to try making sour wort again, I would use the freeze-dried yogurt cultures available at some Whole Foods, Vitamin Cottage, etc type stores. Or failing that I'd use yogurt to start it.
(Dirty secret suggestion: the bugs we like are the bugs we like precisely because they produce basically nothing but lactic acid. A bottle of food-grade lactic acid offers less personal satisfaction, but is a 100% effective way of doing the same thing. It's my opinion, somewhat informed and somewhat not, that the whole "lactic acid from organisms tastes better than acid from a bottle" thing has nothing to do with where the lactic acid comes from and everything to do with having brettanomyces around to make some of it into esters.)