In reply to macstinator: Your best bet will not be individual climbers - postage costs for rope are fairly high.
If I wanted old rope I'd just contact all the major climbing walls local to me. They will get through several hundreds of metres per year and I'd guess they will more than happy to be paid even a small amount for the retired rope. The same goes for outdoor centres.
The only downside is the ropes retired from climbing walls are generally completely trashed and will suffer more from 'blackening' from powdered aluminium from climbing equipment than ropes belonging to individual climbers.