Here's my list of synthetic hair suppliers I've used.
My Little Customs - based in th UK & run by Robin who is awesome to deal with. She sells nylon, saran and now has a small supply of polypropylene hair. You'll need 2 small hanks to do a barbie, but a large hank of saran will do at least 2 barbies & maybe more. BTW... she has a bunch of video tutorials on different methods of rerooting.
DoctoredLocks - Based in the States, these guys sell people size synthetic extensions. I've bought & used the
Janet Collection Yaky Switch Toyokalon Braid, and the
Silky Straight Kanekalon Braid would work too. Stay away from the Kanekalon Jumbo Braid even though it's cheap. There is a reason why it is cheap! Trust me on this.
And here are some others that I know about, but haven't ordered from myself.
RestoreDoll - Based in the States, and sells saran in both small and large hanks.
Dollyhair - Based in the States, and sells nylon, saran, polypropylene, and acetate hair.
RerootDoll - Australia based. They sell saran in both small and large hanks.
If you have a favorite one, please add to the list.