Just got a complete set of new ones with a 2" drop from ESPO Springs and Things
http://www.espo.com/. Ended up costing $500 incl. shipping which is steep but still beats the monos by $400 or more. I can always pull a couple leafs if I want it a little lower with a softer ride.
As you probably are aware, the problem with used ones is replacing the bushings and dealing with the grooves that are worn in the spring--which keep them from slipping on eachother.
If you really want used ones, I've got a complete set I'm taking off my '56--free for the taking. Also have a 235, 3 spd trans. and original rear end I won't be using.