Extract berliner weisse?

I would bring water to a boil, add enough light dme to achieve a low gravity Wort (around 1.040) Chill to around 100F and pitch whatever lacto culture you want. Goodbelly is a good fool-proof source, and can still sour a Wort at room temp. I'd let it sour in a carboy for 36ish hours then do another 10 min boil with it and add hops to get around 5-10 IBU. Chill again and pitch a German ale yeast.

 

You could always just add lactic acid after the ferment to get the sourness, or you could use yogurt and do a kettle sour.

 

@DrewBeechum did an article for BA regarding extract sours if I recall correctly. Probably some good info there.

 

BAM!

All the information that you'd ever want to know and probably more.

 

I'll second that...Milk The Funk is by far the best sour and funky brewing source on the Web!