I am sorry I cant help with any recommendation regards the tank as I live in the UK and dont really know the companies. But maybe send some of the big aquarium companies an email with a link to this thread? Maybe even send some of the companies that make custom tanks...you never know one of them might be generous hopefully.
I think its great what you are doing and I could only encourage you to continue. I hope Gavin continues to recover well. Honestly, my heart goes out to you and your family...I think I can in some ways relate to your situation...My eldest son was born with a congenital heart defect, and has a very complicated heart defect. He has had open heart surgery twice already and is now 7 years old. Still needs further corrective surgery sometime in the future. He really likes my tank and has even begun to understand basic things about the tank. My tank also serves to help me deal with the pressures of life and the moments that I spend in front of the tank with my son are really something.
I hope you manage to sort something out regards the tank. I will keep an eye on this thread. I recently started a Facebook account and maybe I can share this thread there at some point to perhaps spread the word...I dont know if that will help, but maybe something to do.
I have also sent you a PM about a further idea that may help you, but cannot discuss here as it will be against the rules and dont want to get in trouble.