Average shop from what I was reading in the economics in city of 25,000-100,000population had like 10,000g while shop in a city of 101,000+ would have upwards of 60,000 using the dungeonomicon as reference. However I never said there wasn't more then one shop. It's just locating the said item might take a while.
Unfortunately it's not official, but it makes reasonable assumptions. It was good material when it was posted on WoTC before they got rid of 3.5 stuff. I think D&D wiki has a copy of the article.