look on venues own websites.. i'm going to a Lucinda Green clinic at Stockland Lovell.. on their website they have a whole page telling you about what clinics they have coming up!
Find the website for any big centres or XC courses near to you and look on there.
BE have notices for Juniors on their website too.
There are a few eventing taster days with accredited coaches available if you look, there is supposed to be a local one tomorrow but the ground will be like soup, so we are going indoor Sj instead!