There's this game I played on my ds many years ago called Lost In Blue and that series is only available on DS. I really want to play it. Is it easy to find a molded DS?
Lost in Blue came out on the original DS. You could always get a cheap DS lite and a flashcart off of Ali Express.
If you want a 3DS you may be able to find a pre-modded system but I would avoid paying extra for it. The whole process doesn't take very long and there's a lot of good guides out there.
Also you'll likely get random mainstream games when buying a pre-modded system. They won't have every game out there due to the size of 3DS and DS games (compared to something like NES ROMs) so you'll likely end up having to do a little bit of work yourself.