I think the main problem (if you can find one) is that the majority will still be attached to boot lids. Obviously they can be removed, but this isn't the easiest thing to do successfully without causing damage to the spoiler and also to the boot lid itself due to the bonding used to attach it. I have three on boot lids but possibly like the majority, would prefer to keep them as one unit should a replacement ever be needed.
Also the 'Omega 3.2 V6' name plate on the boot lids (as is the 2.6) is also now no longer available new, and succesfully removing them from an old lid and reattaching them to a different boot lid in a straight line and in the correct position is very time consuming (as they are separate letters on the later cars). Using the method as they were originally supplied holds them all together with the correct spacing and positioning and takes less than a minute.
So basically, finding a spoiler that some one is happy to seperate from a good used boot lid may not be an easy task.....as I for one would want to keep the lid, spoiler and name plate all together as one unit.