I guess you could contact the groups that provide . I've never thought of the retired ones ....I thought they were with the families forever .
Once they're too old to do their job, usually they are retired. Often the partner keeps the dog the remainder of it's life, but sometimes that's not possible (especially if,for example, the organization where the person wants to get his next dog from requires that there be no other dog in the house). In that case, the best situation is for the dog to be rehomed to a close friend/family member so that the partner can visit periodically, but sometimes this is not possible either. In THAT case, the dog will go back to the organization that trained it and the organization will rehome it.
That said, it's going to be very unlikely that you'll find a suitable dog, since there are so few available. It's also extremely possible that the dog will have chronic health problems that the adoptive home will be responsible for. My organization recently got a retired dog back, the dog had seizures and other health problems; he actually died about 6 weeks after leaving his home.
IMO, your best bet would be to find a rescue group that has older dogs. My friend rescues elderly dogs from elderly people who go into nursing homes/hospitals/etc., and it is very difficult for her to find adoptive homes for these dogs.