There have been many UG labs making it more than 15mg. Most only goes as high as 25mg. Going higher than that does not make a lot of sense especially for female users, which I guess is not really a huge market of it anyways. The 50mg are offered by a few of the big powder suppliers. I have never used them, and have never seen them tested gilly, but I know that they have been around for quite some time.
Cheers,
Mavy