To answer your original question, no, there are no forums for puppy buyers to post want ads with requirements and get responses from reputable breeders. You can advertise in a number of places, but will probably just attract sellers who are only interested in getting rid of puppies, as opposed to finding you your perfect match.
I would also like to add that it's probably more likely to get a near duplicate of your perfect dog from a repeat breeding of it's parents than to try to breed your own. Plus a helluva lot cheaper and less work.
You might have to breed a number of litters from your dog to get one that suits your bill and that's going to start adding up in costs, labor and time lost. If the litters are small, have genetic problems, or if the bitch fails to reproduce, or dies, you're back to square one after considerable expense, both financial and emotional.
If you're dead set on trying to breed a replacement from your own dog, I would talk to a breeder about getting a pup for a service dog on a limited registration/co-ownership, ask if they would be willing to re-evaluate the dog when it's mature with a mind towards breeding, and if so, lift the limited status. If, at that time, the dog proves to be worthy, I would ask the breeder to mentor me in finding a stud, etc.
One thing you haven't mentioned, or I didn't read, is what type of service this dog will perform. While Newfs are used as service dogs, they may not be suitable for certain disabilities. Their size, strength and maintenance requirements may or may not be a detriment.
I can tell you that the most difficult part of a Landseer to keep clean is his legs. The white on his back tends to stay white without a lot of trouble, but those feathers...sheesh! I think a black dog would be a better choice maintenance wise; however, a dog with more white is easier to see in the dark, should that be helpful.
Lastly, I have yet to see a large female Landseer, more black than white, with a big blocky head. Female Newfs are not supposed to be blocky or large like males and black patches on a predominately white background is the preferred coloring, so these would be things a dedicated breeder would be trying to reproduce. A course, mis-marked female is more likely to be sold on limited registration.
