You can get it from http://www.greenpois0n.com
As of last week, Chronic team had officially release the Mac version of greenpois0n, and the Windows version few days after.
This greenpois0n jailbreak allows you to jailbreak your iDevice(all supported) with iOS 4.2.1 untethered.
This tool is simple to use. Off your device and close your iTunes before proceeding with the jailbreak.
TIPS: You might want to have a fresh restore before jailbreaking.
NOTE: User who had jailbreaked 4.2.1 using redsnow monte can just run greenpois0n over it. It will still work.
Right now, the Cydia store is being flooded with lots of traffic, and so some of the sources might be unable to work.
Its best to wait a few more days before jailbreaking.
If you need help. just post it below and I will try to help.