Your best bet is honestly to contact Robert Dover. He is a super nice guy, just fire off an email. He has a program/website right now for riders looking for owners and syndicate ownership which is a really great idea. ExperienceDressage.com is the site.
I'm in a similar boat right now. I have an owned horse to declare for PAG 2015 on but am looking for an Olympic prospect and having to sell my current horse who is very confirmed at small tour and schooling all the YRGP movements.
If it were possible I would love to have a sponsored/owned horse but my current horse is as sale-able as ever and to get to Europe and buy what I need for Tokyo I have to have the money :( Very sad as he is my fur baby..
First thing to know is that you must be willing to make a lot of sacrifices to make it to international competition, and everything must always be for sale, or sale-able. Sponsorship on a large scale is extremely difficult to come by unless you come from money or have a very good family friend with money - my advice is to marry rich!!
They say money doesn't buy happiness -- well happiness doesn't buy horses!