Here are some links that might be of use. Add some more that you know of to make an ongoing list.
Mods might want to make this a sticky.
This site hold all the rules, Dressage Tests and competitions.
http://www.fei.org/DISCIPLINES/PARA-EQUESTRIAN/Pages/Default.aspx
http://www.equestrian.org.au/default.asp?MenuID=Sports/11757/0,Para-Equestrian/11738/9224