Check eBay and Amazon. Just check reviews to make sure they are genuine.
Also, the Note 8 and Note 7 E-learning are exactly thr same. The only difference is the color. The Note 7 black S-Pen has a metallic black cap, and the Note 8 black S-Pen is a glossy black cap.
I bought a brand new black Note 7 S-Pen for $10 to keep as a back up. That and it came with the replacement nibs and tool, which by themselves cost $15 elsewhere.