I'm not sure how the emus work on n810. Basically, you need a .gba for gba or .nes for nes or whatever. Basically, google will help (the sites are generally paid for and stuff like that. So, find the exact name off the site and then you should probably download it off a torrent) There is no such thing as a virus in a ROM so, don't worry. There could be a virus in homebrew but only if the developer designed it that way... but that's different.

A good site is www.romsite.net it's only DS and GBA but, lets U download one free a day (and more if you know how to use a proxy correctly and get around download issues) or, just find info on the games and then use a torrent.

Technically roms and emulators are legal. For roms though, only if U own the cartridge. It's using them together that is debatable. It's not illegal but it's not legal. Don't worry, game companies don't care with older systems. Even if they did, the only thing the law allows them to do is get you to pay for the game (if you don't already own it)