The biggest and most reputable is Scott's. When I collected they were about 4 volumes. Maybe more by now. My local library also had them as reference books (you could not check them out) so if the total price is more than you are willing to pay you might look there.
Bear in mind that the price Scott's shows is sort of like the "list price" on a new car. Nobody ever really pays that amount unless it is super rare. And if you go to sell them to a dealer you will get far less than that. But it will at least show you which ones are more valuable than others by comparison.
"Laissez-faire capitalism (AKA The Great Material Continuum) is the only social system based on the recognition of individual rights and, therefore, the only system that bans force from social relationships." ~ Ayn Rand
