I've done a small one once. Mashed about 1# pale ale for about 20 minutes, didn't sparge, and let it sit (wort only) covered for a few days in the basement. I then mashed about 10# grains (including specialty) for the normal time, added the sour mash to it, and went through the remainder of the steps. The beer turned out ok, but I would recommend using the cultures. You get a better level of sourness (can control how much culture you add) and you know what you are adding. Unless you live near Brussels (Lambic area), you have no idea what the microflora is in your area. For me, the cultures cost about $10/100ml and I think they are worth it when making a quality sour beer. I would spend the 10 bucks and have a good sour beer instead of doing the sour mash again. My 2cents worth here.