da

on yorehab price guide's front page they always list a few items that are recently jacked (hot)
other than that, i don't think there's a way to see precise % of demand and supply for each item.

^^What Veronica said.

Also if you go in game and check out the auction house, what's being sold will give you an idea too.
If you see tons of a certain item being sold for a low price, it's (obviously) not popular. And if you see something being sold for an outrageous price--but nobody is bidding--that is also a sign of an item people don't want (at least at that price).
As for the supply aspect, i think that only BVG can truly know the answer to that question, since quantities aren't sold with a set numbered amount (like they would if you bought something off, say the Home Shopping Network, for example). Plus you need to take into consideration the Yo-hoarders and their buying habits!