Nintendo DS Games or NDS games are product of Nintendo. You can buy NDS game chip or NDS games at Nintendo Retailer Outlet near you or you can also buy it online via ebay or Amazon.
Downloading Nintendo DS Games is another option but I have to warn you that Download of Free Nintendo DS Games or NDS ROM is illegal as it violates the copyright law and property rights of the game owner and developer.
There are many sites that offer free download of Nintedo DS Lite games and you can start finding those by using the content search of Google, Yahoo, MSN or other searh engines. By typing the keyword “Download Free Nintendo DS Games“, you’ll find the sites where you can download your favorite nintendo games.
Here is the Nintendo DS Games Free Download Page where you can read and find Nintendo DS Game reviews and NDS Game demo so can grab it, plug it in your NDS memory adaptor and start playing the NDS game demo before buying the original game chip.