I found a site ages ago with many old school Winamp anime skins. I'm surprised the site is still up. This dates back to around 1999. While I never used skins on my Winamp, looking through the site gave me a warm fuzzy feeling.

https://niko-niko.net/winamp/

If you're not aware, you can still use Winamp today but it's being kept alive by WACUP. I use it daily!

https://getwacup.com/