Hi and welcome to KF. Technically all kavas are wonderful for anxiety to some degree, part of the fun in the beginning is finding the right one for you.
I don't think Micronized kava is the way to go do to the extra fiber and plant material you have to swallow, medium grind is a much better way, with far fewer gastrointestinal problems than Micro. Nevertheless, if you are intent on Micro, there are only a couple vendors currently selling it.
KWK Pouni Ono is a lighter to medium kava and probably the best one. Cactus kava is the only other vendor I believe, selling micro at the moment.
Our source.