With all the questioning of commercial sources of p-lambic cultures going on, it seems the The Anarchist's p-Lambic is an idea whose time has come (pardon my immodesty). If you feel that buying cultures from commercial vendors is a crap shoot, try this approach (at least you'll have only yourselves to blame; don't blame me I haven't tried this, I am only advocating it).
Here's the idea. You really only need three things to accomplish a passable p-lambic: Sacchromyces cerevisiae for the main ferment, a souring bug for lactic acid production, and Brettanomyces for secondary fermentation (superattenuation and production of funk). How do you get these on your own? Simple. You need two non-lambic Belgian beers that appear to be widely available: Dentergem's Wit and Orval.
Dentergem's seems to come with a nice souring bug (L. delbrueckii) - just ask Scott Bickham who has used it quite successfully. I know from tasting one of Scott's wits that this bug is capable of some intense souring. The Dentergem's yeast will work fine as the primary fermenter. Use the dregs to make a starter. Making the starter well in advance of brewing day will likely improve the souring ability, as it appears that the levels of the bug vary from bottle to bottle. However, with time they seem to increase in number. Be a big culture pitcher on this one.
Now for the Brett culture. The dregs of Orval generally contain several strains of yeast. One appears to be an ale yeast used for primary fermentation. There may also be some slight contamination with a yeast that is probably a brewery contaminant (but don't worry about it). Finally, the dregs appear to contain a really nice Brett culture consisting of several strains of Br. bruxellensis. Again, grow this culture well in advance of pitching. Grow a large starter (1 qt) and let it sit for a month or two. You should then be able to detect the Brett by its odor (vinous, cidery and acetic).
Make the wort in a fashion similar to that of a Belgian wit (leaving out the oats, coriander and orange peel), or like a lambid - I am talking 30-45% on the unmalted wheat. Use conventional hops, but shoot for less than 18 IBU (you may not need hops, but hey it's hard to completely break with tradition). The low IBUs allow you to get around aging and/or baking. Forget the turbid mash, you don't want to spend months waiting for the Brett to break down starches and converting polyphenols. However, shoot for a more dextrinous wort than you would with a wit and maybe a slightly higher SG (1050?).
Pitch the dentergems and allow for a relatively warm fermentation (low-mid 70's), as this may help the lacto do its work. Once the gravity gets down to the mid 1020s, pitch the Orval culture (which should by now have a very distinct odor that is atypical of S. cerevisiae). Conduct the secondary fermentation as an open fermentation (or semi-open) in a food grade plastic bucket.
My bet is that the beer will be sufficiently sour within 3 months (but we'd have to ask Scott to be sure). I think that the Brett will form a pellicle sooner with a good wort/air interface. This may also improve acetic acid production and aroma qualities.
There you have it: a p-lambic fit for Rube Goldberg! It may seem like a real crap shoot, but at least you've gotten to drink about $10-worth of really good beer - something you don't get with commercial cultures. Moreover, you will probably end up with a p-lambic that may be no worse than what many homebrewers produce using commercial cultures.