Finally found Leonardite
I've been looking for leonardite to use under my substrate since I've used it before with great results and it's also recommended by Tom Barr in his non CO2 approach. Unfortunately as anyone who's tried to find some knowns, this stuff is really hard to buy for the last few years.
Well, I've finally found a product that's pure leonardite and fairly easy to find. It's made by Down to Earth and it's called Granular Humic Acids. According to the MSDS sheet it's pure oxidized lignite which is leonardite according to Wikipedia. It's pretty cheaply priced too which is nice.
Hope this helps someone.