This page lists methods of obtaining a legitimate copy of Chrono Trigger, starting with the easiest ways to get it and ending with the hardest.
Ways to Buy Chrono Trigger
If you have a Nintendo Wii or Wii U, you can buy Chrono Trigger from the Virtual Console. This version was released in Japan on April 26, 2011, in the US on May 16, 2011, and in Europe on May 20, 2011.
If you have a Nintendo DS, you can buy the DS version of Chrono Trigger, which was released on July 2, 2008.
There was a PlayStation version released in 1999 in Japan and in 2001 in North America that you could probably find on eBay or in used game stores.
The original SNES cartridge was released on March 11, 1995 in Japan and on August 22, 1995 in North America. It can be found on eBay and occasionally in used game stores. To find out how much it is likely to cost, check Chrono Trigger at PriceCharting.com.