Finding other sites sitemaps.xml files

There are a couple sites that I would really like to see their sitemap xml file. Not the publicly facing html pages, but I would like to see the actual xml - any suggestions?

they aren't listed in the sites robots.txt and if they are they seem to be inaccessible to despite any variations on user-agent I try.

thoughts or suggests?

try adding variations to the sitemap extension to stumble on it (t's not always an xml file)
some alternatives:
domain.com/sitemap.xml
domain.com/sitemap.aspx
domain.com/sitemap.htm
domain.com/sitemap.html
domain.com/sitemap/
domain.com/sitemap/index.html
domain.com/sitemap.cfm
domain.com/sitemap.php

alternatively try google search for "site:domain.com sitemap"

Not all sites run a sitemap, it's just "good practice"