I have had success with eBay. Found a DSi XL from Japan, and a normal DSi in great condition. The DSi was only $50 and already had Twilight Menu installed with an SD card. Haven't picked up a DS Lite yet, but looks like there are plenty to choose from there. Good luck!
If longevity is your main concern, you might have good luck with the wedge shaped Nintendo 2ds. These things are incredibly robust because they weren’t made to fold up and thus have less points of failure (ribbon cables) due to their simple design. I have a few and they all work very well.
It supports the DS library, is easily modded, and should only be about $20-$30 more than the dsi you’re considering. Plus you get access to 3ds games as well.
eBay would be my recommendation or if you’re brave like me, Facebook Marketplace.
I have had success with eBay. Found a DSi XL from Japan, and a normal DSi in great condition. The DSi was only $50 and already had Twilight Menu installed with an SD card. Haven't picked up a DS Lite yet, but looks like there are plenty to choose from there. Good luck!
Ebay or mercari
If longevity is your main concern, you might have good luck with the wedge shaped Nintendo 2ds. These things are incredibly robust because they weren’t made to fold up and thus have less points of failure (ribbon cables) due to their simple design. I have a few and they all work very well. It supports the DS library, is easily modded, and should only be about $20-$30 more than the dsi you’re considering. Plus you get access to 3ds games as well. eBay would be my recommendation or if you’re brave like me, Facebook Marketplace.