Your best bet for finding a Golden Irish for sale is from a breeder or maybe even in classified ads or craigslist. Keep in mind that some people may not know the Golden Retriever Irish Setter mix is called a Golden Irish, so you should look for listings advertising "Golden Retriever Mix" and "Irish Setter Mix" dogs for sale too. You can try rescues, but it's not very likely that a standard rescue would be able to identify a Golden Irish correctly.
More than likely you will need to find a Golden Irish breeder to find a Golden Irish for sale. Since the breed is not a high demand breed you may have to travel to find one with a puppy available and even then you may not have the option to choose which puppy you want. Because of this, it may be wise to ask the breeder place you on a "waiting list" so you can get first choice of the next litter. This may sound like a lot of work, but generally breeders that breed low-demand dogs do so purely because they are passionate about the breed and this results in incredibly healthy litters which truly represent the breed's character.