I see Lindemann's lambics pretty widely available, but I wouldn't want that to be your first experience. It is back-sweetened and like nasty sodapop.
Look for Duchess du Bourgogne. I have had several of theirs and liked them all, but had to go to some fairly pricey bottle shops to get it.
It still doesn't hold a candle to the ones my club produces, though.
Sour beer is inherently difficult to distribute commercially. It is best done in small batches and frequently re-blended to produce the right flavor.
Sorry for the bad news. I hope you find several opportunities to try different stuff. Don't try one and assume that it is similar to what is out there. There is a huge range of flavors and styles of sour beers. It is almost as complex and varied as the non-sour side of things.