You can put a few ounces of crushed 2-row in something such as a thermos and add 150-155F water and let it sit for a couple days. Overnight may even be enough depending on how much souring you want of the mash. You can also pull of 18-20oz of fermented wort and innoculate it with brett, but that would take several months for the brett to ferment out, not to mention you would have to pasteurize it before returning it.
If you just want sour, you could also just steep some acidulated malt with your other grains or PM.