Even if you have NO cash you can play Nintendo DS and Gameboy Advance games. What you need is a PC, some NDS roms or GBA roms and No$gba emulator program. NoGBA will let your PC 'pretend' to be a game console and so you can play DS games with NO ca$h. www.no$gba.com. Emulator could be called NoNDS or No$DS, but as it was started as a Gameboy Advance roms emulator and only later acquired ability to play DS games it continues to be called No$GBA and not No$NDS or anything else like that.

Play NDS ROMs from Internet
Of all the NDS emulators No$GBA has the best compatibility with commercial DS ROMs. And of course it supports running Nintendo DS homebrew.