There are a couple of sources on ebay, from Turkey. I had reservations about it being a hassle ordering from there, but it isn't. They get sent by registered mail and arrive in a week or ten days with no problem, other than having to sign for the package when it arrives.
The ones by Ozgur, or whatever his name is, come with a special cork thingy at the base that replaces actual threads. It's self-threading. He says that meerschaum is fragile, regular threads won't last, but his will. If you want a larger bowl than normal--which to me are way too small, I found out the hard way--contact him directly and he'll make one to order. At least that's what he told me. He's quite English-literate.