Availability seems to be very limited right now, to put it mildly. As the family covers a wide range of capacities, the prices vary accordingly, from about $13 to $373. You don't want to use a device that costs 10 or 20 times as much than the one you need, do you?
Realistically, you should have an idea of which device in which package suits your needs. You may chose to prototype with a larger part (provided it is pinout compatible) if the larger part is available earlier. But before doing this, read Chapter 7 of UG385 (density migration).
To come back to your original question, the only part with limited near-term availablity from Avnet seems to be the xc6lx45-2fg484c (61 available next week, if you believe the popup window).
