Thank you for your post and welcome to the forum. Yes it is true that yew clippings are used for medical research and for the treatment of cancer. The yew plant or Taxus baccata contains a cancer fighting chemical. The clippings are normally collected between July and September and they normally only require 1 years growth.
I have had a little search and have come up with a few sites you could try :-
Here is the first one http://www.limehurst.com/
And the second one http://www.friendshipestates.co.uk/yew-collection/yew-collection.html
I hope that helps and answers your question.

