It would depend on many factors, but yes, there are 100% peaberry coffees. Technically, peaberries develop with only one bean per cherry, and are typically oval (like a pea), where most coffee develops with two beans per cherry and are flat on one side. Many mills use mechanical, gravity, or screen methods for sorting sizes. For mills that use this technique, the 'Peaberry' designation is in reality a grade, and not necessarily 100% peaberries. For this to happen an additional hand sorting is done making 100% peaberry coffee possible.
This would be a good question to take to your importer before you buy a lot.
I saw a good writeup on this awhile back, think it might have been Kenneth Davids...will see if I can find it.
I often get 100% peaberry Mokka from Maui. Cute little suckers. But I would not recommend spending the extra 20% or so that many people charge. I charge the same price as the regular Mokka.
I agree with the article mentioned above. Peaberry cups differently, but not necessarily better. I hate it when I walk into a store and they try to sell me peaberry because it's so much sweeter than other coffee.