i dont have exactly what you want, but i do have some argillite to trade. it's a bear to knap, most guys wont touch it but it was common in pa, wv, md. i quarry it myself at a 10000 yr old quarry along the delaware river. i could send you a medium flat rate box in trade.
tell me more about the plant fiber slings. if interested
here is some of my argillite work. i use the kelt and knife for hunting and have thrown and stuck the kelt with no damage