Ebay isn't a bad starting point, there are 12U cabinets on there for less than £200 and most of the sellers are retail places with their own websites that you can go to for a more organised catalog than you get from Ebay.
Cost is partly driven by the size but also the depth of the cabinet, so bear that in mind. Some cabs are only 300mm deep which is basically just for network kit like switches, 500mm will allow you to fit smaller server like Dell R230, 1000mm will i think allow for the really big rackmount servers. However, there's no point buying a deep rack if you're never gonna put anything bigger than an R230 in it.
I got the 12U 600x600 version of this for about £90 a few years back:
https://www.linx-com.com/products/c...s/wall-cabinets/wall-mounted-cabinet-model-e/
It's not big enough for large rackmount servers but with a few shelves fitted it's been perfectly fine for for my Microserver, a bunch of USFF machines on rack adaptors, plus some rackmount networking kit and a rackmount NAS. I've also got a rackmount drawer for miscellaneous crap like spare cables and tools and stuff. Occasionally I regret not getting something deeper but then that's probably stopped me wasting money on massive electricity-guzzling servers that I probably don't need anyway
What I will say though is if you aren't planning to get rackmount gear I would seriously question why you want a rack. For a couple of tower machines there is not a lot of value to having one, and you will have to worry more about cabling not to mention things like airflow for cooling. In addition to the rack itself you will probably need to think about a rack PDU, cable management, maybe some top/bottom fans, all of which can add to the cost.
