Ok I've spent the the last hour searching for it but can't find it, how do you become a supporting member?

 

There are some Supporting Member tags in use, as you may have seen, that date back to a time when some members gave some funds to ThirdGen when it was owned by one of the original founders. It was a method to show support and also help pay for server upgrades. This was over 10 years ago.

Right now, there is no official paid membership option. It's possible it could be an option at a later time.