Your best bet is probably Amazon. It appears to be a 3rd party seller, but it's shipped and covered by Amazon should any issues arise.: https://www.amazon.com/Samsung-Galaxy-Factory-Unlocked-Smartphone/dp/B06Y14T5YW/ref=sr_1_1?dchild=1&keywords=galaxy+s8+new&qid=1603940188&sr=8-1#customerReviews
You didn't specify if you need an unlocked phone, but if you can get away with a Simple Wireless one, Walmart carries them on their service still available online as well as the S8+. Newegg also has several 3rd party sellers - may not be as risky as eBay though your mileage my vary.
Walmart and Amazon both have them listed. I bought my S8 after the s10 was released, so they made em for a while. Gotta just look around, there's new ones available.
Edit: Not sure who downvoted, but a quick Amazon search shows in-stock right now. Also, new available doesn't mean they're still being manufactured! They're still available on some shelves: https://www.amazon.com/Samsung-Galaxy-Factory-Unlocked-Smartphone/dp/B06Y14T5YW/ref=sr_1_1?dchild=1&keywords=galaxy+s8+new&qid=1603940188&sr=8-1#customerReviews