So, take what I have to say with a grain of salt as I haven't brewed a sour beer before (or done that much brewing at all, to be honest). The sourness from a lot of Belgian styles like lambics, gueuze (mixture of lambics) and other wild ales comes from the bacteria that "infects" the beer. Bacteria such as brettanomyces and lactobacillus are two common ones that I know of. The bacteria can get into the beer after boiling by letting it cool naturally (without a chiller or ice bath) and letting it sit exposed, and also by letting it sit in oak barrels where bacteria is living and nested into the wood.

If you're homebrewing, I'm not sure what the best way to infect your beer would be. If you let it sit exposed after boiling, I'm taking what is a rather obvious leap (I think?) and saying that the bacteria you're after may not be what gets in your beer, though I could be wrong. I know Jolly Pumpkin lets all of their beers sit and cool naturally and they let bacteria from the air fall into it, so I could be wrong. They also let all of the their beers sit on oak where bacteria from the last batch is still alive, so the more the barrels are used the more sour beers they can produce.

My two cents, maybe someone else can clarify a few points.