Special schools are increasingly being closed in favour of placing their students in mainstream schools, so as a teaching assistant in a mainstream school, you are very likely to be working with children and young people with these conditions, as well as children with a whole range of other difficulties. Training and funding for such posts is usually administered by the Special Needs teaching service of the local education authority, and you could do worse than give them a call and ask their advice.
The way into adult teaching with this client is more difficult, as colleges tend to recruit qualified and experienced staff. However, I do know of one guy who began at our local college as a teaching assistant and is now a course leader there, having qualified on the job (he had a degree, but was able to work towards a teaching qualification). He began by covering for the course lecturer on occasion, gradually moving up to having his own teaching and assessment responsibilities. You could possibly make a start with this client group by going to your local college and offering to volunteer.