Yeah, don't buy a battery from someone claiming an 18650 has 8 v.
You probably saw a Nitecore battery that 8 AMPS, which in general isn't enough for vaping.
As mentioned, you should look for a reputable manufacturer like
@Syythe listed, probably 20 Amps or up, and also a reputable dealer. Some that are pretty trusted include Liionwholesale.com, IMRbatteries.com, illumn,com, and 18650batterystore.com.
For any REAL battery knowledge, google your question, and start with the name "Mooch".
(ie. "Mooch which 18650 last longest")
Mooch is THE trusted expert on batteries for vaping, and he's probably got a blog posted somewhere with the exact answer to your question.