Newer firmwares for Cisco, Juniper, Brocade, et al, are a little harder but still easy to find.

In both cases caveat emptor, since you have no way of knowing if they're trustworthy or not. I messed around with some older yarrrrrr firmware when I was studying for my CCNP and built a lap from craigslist hardware, but that was behind a pfsense firewall I built myself and kept totally away from anything that mattered. No malicious behavior AFAIK, but I'd never trust those IOS/IOS-NX images in Prod.