I've been having the same problem, the supply of all GeForce3 Ti500s seems to have disapeared. I am having a system built for me, and both the fellow doing it as well as myself have been looking for cards for over a week without any luck. They don't have it in stock, they can't order it and they have no idea when things are going to get better.
I am getting the impression that the GeForce4 is causing manufacturers to dump their inventories, and that they underestimated the demand in the short term. It is stupid, as even with the GF4 impending I need the machine now, and would be happy to pay the price - however I don't think anyone wants to be left with inventory of $400 cards when they are eclipsed and their price will fall.
I am probably going to be just buying an inexpensive GF2Ti200 card with fast RAM and live with that until I can get either the GF3Ti500 or the GF4 - depending on the rumored announcements and how well I can survive with the little card.
If you hear anything pass it on though

Thomas Sapiano,
tsapiano@ieee.org