Ah, good question Steve. There are a couple of good sources and there have been a few comparisons done here on the site showing who has what pearl.
Ofrei has good ones, and another company (PMWF?) has 'em too... Freddy333 is a good source for that info.
There are also pre-aged pearls available on Ebay, where the back is dug out and nasty brown speckled goo is pushed in, so it looks like an authentic aged tritium pearl. They're about $25 each, I've bought a few and they look great.