I have done the method using raisins to make a sourdough starter. Essentially soaking ORGANIC raisins in water for a few days and then using the water from that to mix with flour. The idea is that the skins of the raisins (grapes) have wild yeasts on them. How much of that is true and how much comes from the air is uncertain.
The question I have is what is your aim? Are you looking to make a wild beer? Sourdough bread is a combination of yeasts and other microflora. Specifically strains of actobacter and lactobacillus. How wet or dry your starter is will determine the dominant bacteria. IIRC wet (i.e. more liquid starter) favors lactobacillus, drier favors the acetic (thus more sharp sourness) bacteria.
I know some homebrewers are very paranoid about keeping sourdough starter and brewing beer in the same house. Many people have warned me not to do it! I have been brewing beer and making sourdough breads for about 6 years. So far so good. In fact I have had a starter for much longer than I brewed.
If you are interested in wild beers read the book wild brews by Jeff Sparrow.
If you just want to muck about blindly, go for it. No harm in trying. Make up a bread starter and try it.
Or perhaps you can use some milled malt. Soak it in warm water and leave it a few days. It will make a microbial culture of one sort or another. Whether it contains any Sacchromyces strains or not is questionable.
I have played around with making starter for a Korean rice drink known as dong dong ju. It involves soaking milled wheat berries (unmalted) and allowing them to grow a mold. The result is known as nuruk and it serves as an enzyme (similar to koji kin, or Aspergillus oryzae used in sake brewing) to convert and ferment the starch in rice. It is an acquired taste...
I believe any starter you make with bread flour will favor yeasts that are better suited for making bread. Bread yeast is in the same family as beer yeast. However, as there are different beer yeast strains with different characteristics, so too does bread yeast have its own peculiarities. As far as I know it tends to be less alcohol tolerant and thusly produces less alcohol. It tends to be more geared towards producing CO2 which is more important in bread production as that is what leavens the bread. Also, remember a sourdough starter is a mixed culture of yeast and bacteria. So if your intention is to produce a clean tasting beer you will be dissapointed.