As WilliamTell mentioned above, Japanese labels including denim are are very niche market. Sure, they seem extemely popular on the boards like SuFu, but that only represents a fraction of the population, and an even smaller fraction of Americans.
You mentioned Proper in the LBC...if you are talking about the sneaker store that is all they have, but if you are talking about their clothing venture, you have mistaken (they are my homies and I love them to death...but they stock some terrible stuff IMO ie The Hundreds, Acapulco Gold, Crooks & Castles, etc). The Academy and Seams never stocked Japanese labels.
IMO, if you were thinking of starting a store in the LBC I would be extremely wary. Those who know what they are looking for will make the trip to LA to Union and Self Edge, and or to OC to Blends...to more established and reputable stores.
You realize you are answering a thread from 2009