guess you've found it now, but in 2007 it was around $25 extra per person, and for that you got to use a shorter queue, once per ride.
as far as i'm aware though, they only give out a limited number each day, and sell them from the gate, first come first served.
little tip if you use disney.....the fastpass tickets don't differentiate between kids and adults. you just put any park entrance ticket in the machine and get a fast pass out.
we used our 4 tickets (2 adults, 2 kids) to get 2 fast passes for 1 ride, and 2 for another......my wife and daughter then took all 4 tickets and did the 2 rides, while I took my son to a show.
it meant we were seperated for 30mins, but my daughter got to do the big rides much quicker.