Preferably one without a power supply, or ... literally anything inside. I just want the case.
Honestly, a busted up, broken router or broken switch would probably work too.

I'm wanting to build a rack mounted, Raspberry Pi Cluster/Pi-NAS.
Not sure if it needs to be 1U, or 2U. I *THINK* you can fit 3.5 HDs in a 1U, side by side.

I want to be able to fit, 3-4 Raspberry Pis into it, and two 3.5mm Drives, and a fan or two.

I'd rather buy used, because I really don't want to spend a bunch of money on something that is essentially just a sort of hobby project for me.
If I can find one for dirt dirt cheap, or even free.... I'd appreciate any help :)

I got rid of my extras a little while ago unfortunately.

Supermicro usually has cheap used chassis for sale, but you get what you pay for.
https://www.ebay.com/sch/i.html?_from=R40&_nkw=supermicro+512&_sacat=0&_sop=15

You should be able to fit what you mentioned, but it might get tight. Those short chassis really don't have too much room.

I suggest using a regular PC case if you can get by without the rack mounting, but I understand wanting it as well