Jewelybabie - the link for the clasps that I used is here, although I have also used the vendors Caitlin listed:
https://www.rings-things.com
Go to "our products" in the menu on the bar on the left hand side of the page, then choose "finishing findings", then "clasps" and then "magnetic". The ones I used are on the last page. That vendor is also where I got the clamshells and split rings. They call themselves wholesalers, but they sell to folks (like me) who don't have tax IDs. I think they have a $25 minimum order, though.
Ana - yes, the prices seem outrageous at first. Ni`ihau shell lei, like the ones you linked to, are insurable fine jewelry. The cost is a reflection of the rather arduous collection/cleaning/stringing process. I collected the shells for this bracelet on 3 different trips to Hawai`i. In that time, I collected about half of a baby-food jar's worth of shells.
Sand gets wedged into the opening of the shells by the surf, and must be pried out by hand using a pin or awl. I would hold the shell between my thumb and finger on my left hand, and try to pry out the sand grains with a pin in my right hand. The sand doesn't dislodge easily, and I ended up sticking myself in the fingertips frequently. It takes a lot of force to dislodge the sand grains, so when the pin "jumps" it jabs pretty deeply - I lost a fair amount of blood from my fingertips!
I would clean out as many as I could stand and then let my fingers recuperate for a few days before cleaning out another batch. Many of the shells I just gave up on - they remain sitting on my shelf, mocking me, with their sand grains firmly entrenched in their mouths. However, I did manage to clean out the openings of several hundred of them.
Next, an awl is inserted into the opening of the shell and used to punch a hole in the back of the shell for stringing. Many shells shatter during this process. Of the ones that remain, they are sorted by color and size (I did the best I could with my limited stash - the shell lei artisans have a much larger stock of shells and can do a nicer job of matching). Finally I was able to string together the bracelet (after reaming out the pearls to accommodate the 4 strands of silk).
The bracelet is truly a labor of love, and helped me to understand why the Ni`ihau lei cost so much. My shells came from a beach on Moloka`i which has special significance for the intended recipient. The actual Ni`ihau shell lei use the same type of shells, but collected from Ni`ihau island (and to a lesser extent, Kaua`i). They are supposed to have a much nicer luster than those collected on other islands.