if some company or organization already runs one, i'd rather use that
I'm not sure that's a good idea. The benefit of
example.com is that it is standardized. You can be reasonably certain that it will not change. It will not disappear, it will not start logging requests (if you trust the IANA), it will not serve actually malicious content, etc.
You can't do that for any random domain from some random company.
Though if that doesn't matter to you, you could use any of the existing evil.[com/org/etc] domains.
I'd recommend against it though. Alternative ideas:
example.org (both are reserved according to rfc2606). Spell it out that
.org is "evil" and
.com is "good".
- use any of the reserved tlds (
evil.example. They won't resolve, but it may be good enough for you.
- use a fake subdomain of the reserved
example.com domain. Eg
evil.example.com (but again, they will not resolve).
The benefit of any of these is that you can be relatively sure that they will remain as they are (as they are reserved domains/tlds).