While the suggestions that others have pointed out are good, I would like to answer your question with a follow-up question. Do you really need the ROM-files in question or just access to the games?
Many of the popular games are also available on other consoles. For instance, Super Mario Bros. is available as part of the NES Classic Edition and on the Wii Virtual store.
If you don't want ports of the games due to possible differences between versions, then purchasing the original games and playing them on the original hardware (don't forget that modern TVs are not ideal for retro gaming!) is likely the best option. This is, of course, an extra cost but in comparison with salaries and other equipment costs connected to scientific research, probably rather low. Furthermore, depending on what you want to study in the games, ROMs might not be 100% perfect emulationsrepresentations on the effect of the original gameorigninal hardware either. (I am no expert on technology though, so I'd suggest talking to somebody who knows a thing or two about hardware emulation and ROMs if the authentic representationfine details of the game is needed down the smallest and finest details, original hardware performance is likely the best optionof importance.)