I know that they both exist. Moonglows are not pure white, but more of a very light gray. I have never seen one for sale though. The few I've seen, where found by mistake. They were bought as light yellow juvenilles, and grew up to lose all their color and look almost white. (So maybe try buying a few lighter colored yellow babies and see what happens.) Giants do exist also, but I've never seen one for sale. I think these two types of cresteds are so rare that anyone that owns them is holding onto the breeder and their offspring, probably to make a project out of these morphs. I know this doesn't help much. Just wait a few years, I'm sure they will be on the market before you know it!
-Nicole