- Edited
A word about copyright.
Nobody said you should pirate, and just because someone does say, "It is not stealing when it is a backup/preservation" does not mean you got the copyright somehow. But we have a service problem and there are no regulations that tell us how we can keep our favorite games alive to get new life out of them.
A overview of some practices
Buying or re-buying a game in a bundle/store or with a product. Sometimes the games can be extracted and also be used for modding. Some products ship with pirated dumps. Buying from 3th party only supports the last "owner" and probably not the creators.
Making your own backup. This can be quiet a project and some web searches should give some methods how it can be done. There are devices that can transfer save data in case you are nostalgic about them or need to replace a battery. Backups can be played on emulators/FPGA devices or ROM-Flash-Cartriges on original hardware.
There are some archives that host the dumps. Without them some of video-game history would be lost to time. A lot of effort went into preserving and documenting as much as possible and this is all fan labor. I wish I could say the companies of the product would care as much as we do about this games. So we can celebrate them the way we like. Sure we care about new creations, hardware and everything the companies need. We try to give all the respect they deserve. In a perfect world there should be no issue for both to exist in harmony.
If you can't find them I am sure someone can tell you some sources but it is very unfriendly to ask a stranger for a ROM! (It is up to you to decide the route you choose to go!)