The other difference between saltwater cultured and freshwater cultured is the sharpness of the luster. Saltwater pearls have a very sharp, mirror-like luster while freshwaters have a softer, more diffused "glow."
I recently purchased a natural white Hanadama from pearl paradise. It's a gorgeous, metallic white. I didn't want the traditional "pink" overtones with most akoyas. I went and looked at some Mikimotos and was astonished how pink they were! I know people say they look better on the skin, but I don't like pink
I know the nacre thickness is a problem with akoyas, which is why I got the Hanadamas which give me the average thickness in a certificate so I know exactly what I'm buying. Otherwise, the nacre could wear off in a few years of heavy wear (and reveal the mother-of-pearl bead inside). I don't wear mine that often, so I am not too worried.
I plan on trying the Freshadamas at some point since they are SO much more affordable! Shoot, for what I paid for my Hanadamas I could have gotten a much larger mm pearl necklace, bracelet and earrings in Freshadamas! But I am sucker for the hard, metallic look. Shoot, some people can't even tell the difference at all!
My problem with pearls is that there is NO standardization in grading them! Super frustrating! I think that's why freshwaters are a bit more prevalent... you know you are getting solid-nacre pearls and hopefully the luster is as pictured. The Chinese are getting better and better at getting freshwater pearls as round as possible, so I think in a few years most people will prefer the less-expensive freshwaters, especially if they can get that metallic look of Akoyas (I am anxiously waiting for the day!). Round, metallic freshwaters... ahhhh! I've only seen off-round metallic freshies.
I haven't seen anything to suggest that akoyas are harder to find... there are lots of websites out there that sell them. But buyer beware! I looked at one company and couldn't find ANY reviews and they aren't part of the BBB and I questioned some of their pictures of low-to-high quality pearls (really dramatic). Also, most "akoyas" are Chinese now. The Japanese lost most of their akoya beds years ago. Even the Hanadamas I bought (and are certified in Japan) are most likely Chinese pearls purchased by Japan and "re-branded."
I really hope this helps!