DS / 3DS buying

Hi! I recently went digging for my old DS and discovered the top screen no longer works. I'm not sure how much it would be to fix it.

But I'd also like to buy a 3DS/3DS XL so I can play 3DS games too, but I'm completely clueless about this. Where do people recommend buying the consoles from? As I don't want to get scammed for being clueless.

Edit: What about modded consoles?