Chrono Trigger is a Super Nintendo game released in 1995. The Super Nintendo cartridge is now rare and out of print and it's very expensive to buy.
There are other options to buy Chrono Trigger.
You can buy the 2001 Playstation 1 version featured in Final Fantasy Chronicles. But keep in mind this will have extra anime cartoon cutscenes.
As of 2019 you can no longer buy it on the Nintendo Wii shop channel as it has shutdown.
You can also buy it on the Nintendo DS but be warned that the English translation is much different than the SNES version.
You can also buy it on the iOS and Android shop channel.