I have seen occasionally discussion of "sour malt" on home brewing forums. What is it? Are there any commercially available sour malt beers?
Sour malt (aka. acidulated malt, saurmalt) is a base malt that has been processed so that it contains lactic acid.
The acid is useful in adjusting the pH of the mash during brewing. Typically brewers use a 1-10% with 2-5% being typical.
The malt's existence is due to the Reinheitsgebot - the German Beer Purity law. This forbids the use of any ingredients other than malt, hops, water and yeast - making the use of other additives to adjust mash pH illegal. Hence, saurmalt was developed as a way to allow brewers to adjust the mash pH while staying within the restrictions of the purity law.
Nowadays, using a sour mash goes way further than just correcting pH. It's used to define the sourness in some beers, such as Berliner Weisse, Lambics, Flemish Reds and Browns and Stouts.
So, while you can't buy a beer made only with sour malt, there are plenty that have used it, or an equivalent souring technique to produce the sour notes in the beer.