Hm... Yes, quality horn picks are a problem here in Croatia and I guess even bigger overseas. I have a similar problem like you. I have 1 (one) good horn pick for kontra and if I lose it I'm stuck with a plastic pick.
The problem is that there isn't an organized trade of making horn picks and there isn't one or more established manufacturers. Tamburasi make their own picks once they find a horn of good quality and then they give them (sell them) to their fellow tamburasi.
On the thread "prim i trzalicu tražim" there's info about a guy who allegedly sells deer horn picks he gets from Serbia - the prices are - prim 15€, basprim 20€, kontra 25€. His e-mail is
felipe@vip.hr I don't know anything about that, it's just a translation of marek's post about picks. Marek also says that the picks are large, they don't wear off and they can last up to three years (if you don't lose it, lol).
So, maybe you should e-mail this Felipe and get more info.