You can find these consoles wherever you would otherwise find used games or consoles.

If you are in North America, western Europe, or Japan, the DS family of systems was ridiculously popular and you shouldn't have a problem finding one at a relatively affordable price.

Be careful with Amazon but there's no reason to avoid it. It is a marketplace with sellers just like ebay or wherever else. However, it is probably best to avoid their "renewed" program which are sellers who have allegedly "tested, inspected, and cleaned". Notice it doesn't say refurbished, so nothing has really been done to make the console more functional. but they're still charging you a lot for it.

Look at the price of the Pink DS Lite Renewed Vs the Pink DS Lite. $40-50-ish vs $180. Huge price difference.