Pretty much the definitive website on all things II.
Another informative page on II.
A really good PDF (free download) entitled 'An Introduction to Intensive Interaction' by Firth (2013). Well worth a read.
Another really good free PDF download by Firth, entitled 'The Evidence Base for the Use of Intensive Interaction with People with Learning Disabilities.'
Website of Dave Hewett who originally developed II together with Melanie Nind.
Phoebe specialises in II and autism, with a strong element of sensory integration.
Search for Intensive Interaction!
I would also highly recommend the Facebook groups 'Intensive Interaction for Parents' and 'Intensive Interaction Users' - search for it and apply to be a member.