If you haven't encountered it, examine.com is a good resource; they curate research on supplements.
In the Human Effect Matrix, in the Anxiety entry, you can see a link to 3 double-blind, placebo controlled studies examining the effects of ashwagandha on anxiety: https://examine.com/rubric/effects/view/a42299063269c89b14ce...
As far as sourcing, you could look at consumerlab (paid subscription; they do independent testing of supplements).