From knowing a few vets and owning a Rottie myself, I can suggest using vets as contacts. They usually know the breeders and can recommend you someone. I got my Rottie from a vet friend myself and couldn't be happier. One important detail though that I've been able to witness so far is that purebreds tend to be very delicate to local disease. Parvo-virus kills many every year, almost killed mine, and parasites are also a big problem. If your dog will be living with you, then I don't think you should have any problem. But if it's going to get into contact with street dogs, or live in the "campo", then I would suggest a crossbreed. These are very tough and don't get sick as easily.
If you're want, PM me and I'll give you one of my vet friends' phone number.