I’ve been trying to find white moonstone heishi (4mm) beads for a friend who is driving me nuts over this. No luck at all. Usually, most sites carry rainbow moonstones, and the few that have white either don’t carry them in heishi style or do at larger sizes. Is there a reason these are so rare? Does anyone have any leads? Googling isn’t helping. Thanks.
I’m not a jewelry craftsperson, but since you have no other replies I’ll hazard a suggestion.
Mr. Cherry2000 and I never miss a Rennaisance festival (yeah, we’re geeks 
), and there is always TONS of jewerly there made from some unusual materials. Have you thought of contacting some of the types of folks who craft for these things, and seeing if they might be willing to share sources?
Don’t know if the stones are exactly what you are looking for, but he bought me a gorgeous pair of earrings that have approx. 4mm white moonstone beads in them form these folks.
I used to do a bit of costuming and stage-type stuff, and used to have sources for all kinds of odd things. I’ll poke around, and if I find any of my old sources I’ll come back and post them for you.
Try Fire Mountain Gems – they have an assortment of white moonstone beads on p. 107 of their 2004-2005 catalog. (I’m not sure if this is the most current one; I haven’t ordered from them for a while,* so they might not have sent me a new catalog.) There are 4mm and 5 mm rounds, 5 mm irregular “buttons,” and 6x4mm irregular “bricks,” among larger shapes and sizes. Don’t see nay that I would call heishi, but I’ve found that other people don’t always use that term the way I would. 
*No reflection on their service or selection, both of which are excellent; I’ve just been moving away from using purchased beads toward making my own. Standard disclaimers, etc. I would recommend calling or e-mailing them rather than trying to order online, since their selection is always changing.
You could also try Rio Grande (www.riogrande.com) and Rings & Things (http://www.rings-things.com/index.html; requires state resale number) if Fire Mountain doesn’t do it for you. There’s also a large WI bead shop (www.turtleislandbeads.com) that might be able to hook you up if you give them a call. I used to sell my stuff there on consignment and the Turtle Island folks are very nice.
Another thought: Pick up a copy of Bead & Button magazine and check out the gazillions of ads; you might get some ideas of other vendors to try.