eBay, Mercari, Facebook Marketplace, local retro game stores.
Always verify hinge tightness (common failure point) and screen condition before buying.
Better Alternatives for Modern Play
Nintendo 3DS / 2DS Family: Plays all DS/DSi games natively, adds 3DS library, homebrew support, and eShop (until 2023 closure). 2DS models start ~$80 used.
Analogue Pocket / Retro Handhelds: FPGA/emulation for DS via cart adapters or ROMs (legal gray area).
Unless you’re a collector, a 2DS/3DS is the smarter buy for playing DS/DSi games today.