As far as I know, eShop services should still be up, I have no idea why you wouldn't be able to buy games with a credit card.
For 3DS games, you don't even need a flashcart. Follow this guide and you'll have everything you need to get started:
https://3ds.hacks.guide
Old DS(i) games can be run using a tool called nds-bootstrap, but compatibility is far from perfect and depending on your situation you might want to invest in a flashcart for those games.
--------------------- MERGED ---------------------------
Also, laws vary by country. but at least where I am in the USA installing CFW and running homebrew is always legal, provided you're not using copyrighted code to do so.
Using CFW to play pirated games is always illegal, it breaks international copyright laws. As far as legality goes, it doesn't matter whether or not the games are still being sold. The question is whether or not the publisher is going to care.
There are misconceptions regarding "abandonware." You should definitely read Wikipedia's page on it:
https://en.wikipedia.org/wiki/Abandonware
But the bottom line is, just because the owner has abandoned the software, doesn't suddenly make it public domain, and unless they specifically relinquish their copyright, you could still be charged with piracy.
So, would it be legal to download these games? No. But your chances of facing charges if you do are slim to none, especially if you're not redistributing them.