WHERE ARE THE GAMES?
The Retropie site claims that if you have a legal copy of a game then it is legal to download the corresponding ROM. It also maintains a page of links to free and paid legal ROM sites (http://bit.ly/lxf257legal). One of the recommended free ROM sites we visited during this review led only to spammy-looking EXE files, so visit these at your own discretion. Check copyright law in your jurisdiction carefully before downloading.
Retropie encompasses a vast number of emulators from the Amiga to the ZX Spectrum. If you’re not sure whether a particular platform or ROM file type is supported, check the Supported Systems of the Retropie User Guide (http://bit.ly/lxf257support).
If you still have any Sega Mega Drive/genesis or Super Nintendo cartridges, you can access their files over USB via a Retrode. While these are no longer officially being produced, they are available from third-party manufacturers such as www.dragonbox.de/en. You can also buy a separate N64 adaptor.
Second-hand cartridges are freely available on sites like Amazon and ebay. There are also websites dedicated to selling retro games, though some, like Retroscene, have apparently fallen foul of copyright laws. The Internet Archive also hosts a large collection (https://archive.org/details/messmame) of ROMS of arcade classics such as Space Invaders and Pac-man. The website claims that hosting these games is deemed as ‘fair use’ under copyright law, but check if this is allowed in your country (it almost certainly won’t be – Ed).