Yes, archive.org :-D. You can find plenty of arcade games, all running in a browser version of MAME. You can also find games for other platforms, such as home computers or pc, them too running on a browser version of some kind of emulator (MESS, Vice etc.)

Since we are talking about Sega, have some Sonic the Hedgehog.

https://archive.org/details/sg_Sonic_the_Hedgehog_Rev_1_1991...

Not sure if it’s allowed here but yes, people have been dumping even the most recent arcade games: http://www.emuline.org/forum/28-arcade-pc-dump-loader/

Sega is selling the consumer arcade stores. Yes, there are communities that work to preserve arcade machine ROMs and other media.

Regardless of where one stands on warez, it's often the de-facto only source for obscure titles.

There are/were fairly-complete warez torrents of MAME sets. The ones up to the early 2000's were of reasonable size, but after that, the sizes balloon.

Best to get a stand-up MAME arcade which were the rage some years ago.