I haven't gotten into technical culturing, but I have used yeast from commercial beers to brew 5 gallon batches.
I have done this with pretty good results using Deschutes beers. I make a small batch of wort with 6 oz water and 1 tablespoon DME. Boil the wort, cool to 75 deg. Flame the lip of the beer bottle, pour out all but the last inch. Pitch that remaining inch of beer into your cooled wort. Let it sit for about 2 days. You'll see a cake of yeast form at the bottom. I have made this little starter in a mason jar, but you could do it in the beer bottle with a stopper and airlock.
After a cake of yeast has developed, make a one quart starter using your yeast culture just like you would a packet of liquid yeast. Give it a day or two and pitch.