Hmm, the best company? eBay seems to be the most common source.
But be prepared to defend your request. Judging by your low post count, you may be new around here. We look out for each other, here, and do our best to protect each other and the laser hobbyist community. As such, expect us to ask you questions about safety, your knowledge and your level of responsibility.
This is among the most dangerous lasers out there, for many, many reasons discussed numerous times on this site. I encourage you to use the google search bar, and begin reading up what people say and have said about CO2 lasers.
When you begin impressing us by what you've learned, you'll find us more cooperative and friendly.