Well, technically that would be illegal, as most emulators are made for free, and for fear of being sued for "promoting piracy of roms" they always say not to package the emulator with roms or to sell it.
However, I think in parts of Asia where copyright laws are more slack or less enforced, you can actually buy ROM cd's in regular stores.
However, recently, they have offered commercially released roms for sale as emulators. Ie: Buy a hotrod joystick and you get a bunch of capcom arcade roms ( http://www.hanaho.com
) or you can buy one of Sega's "Smash Pack" type things which is basically a modified KGen emu with Genesis roms. Those are 100% legal and pretty good.
And hey, roms and systems from defunct companies are being released by the developers all the time, so you never know!