I assume you mean colocation in the traditional sense, where you rent space along with power/networking, and provide your own hardware. (As opposed to the more turnkey dedicated servers where the host provides the hardware too).
You might consider browsing the WebHostingTalk forums [0], which - at least back in the day - were the best marketplace for finding good opportunities. I haven't been in the market for a long time, but it seems like the forums remain pretty active. You'll want to be careful to watch out for resellers that aren't adding any value, but you can find hosts from every part of the supply chain there, including some diamonds in the rough.
If you actually meant dedicated servers, then I'd still suggest WHT, but find the right subforum.
[0] https://www.webhostingtalk.com/forumdisplay.php?f=131