Love the Internet Archive. They’re my second favorite place to get ROMs, and it’s specially important now that most ROM sites are borderline useless with most links removed.
First place goes to one specific FTP server that contains a clean copy of every single possible ROM, for every system imaginable, with high speed downloads and the correct file extensions. I have copies of their IP address in multiple places, hell, I might tattoo it, I’ll die with this number imprinted on my brain my fear of losing it is so high.