The most legitimate method would be to buy a physical cartridge and dump it with a cartridge dumper. You'll probably need to clean the pins with isopropyl alcohol and sometimes also a fiberglass pen to get them to read reliably.

I don't have a specific cart dumper to recommend from experience, I have dumped GB/GBC/GBA games but not SNES. A quick search found some options, though.

https://github.com/X-death25/SNES_Dumper

https://stoneagegamer.com/retrode-2.html

Also found some discussion here recommending the Sanni Cart Reader, but it's a 5 year old thread so there are likely cheaper or better options.

https://www.reddit.com/r/Roms/comments/obrxg3/best_nessnesn6...

That being said, I do not think there is really an ethical problem with grabbing someone else's cart dumps, whether you have a cart of your own or not, and legally I would be very surprised if you had any issues in the US at least. The coolest part about cart dumpers to me is for carts with save files, you can "rescue" them from the hardware and preserve your progress. In some cases the save relies on a battery that could go dead at any time.

If you do end up collecting physical cartridges, I would also encourage you to get the actual console, and explore mods for it, get the best video signal out of it you can (RGB or component rather than composite). Playing on real hardware is cool. I'd also recommend getting a flashcart even if you do collect original cartridges, so you can try out homebrew, romhacks, fan translations, and ports. There's a guy who has been porting NES games to SNES.

https://archive.org/details/@infidelity

Super ZSNES – GPU Powered SNES Emulator