Here is the link for the Club in the US.
http://www.amstaff.org/
Start there. Never buy a breed such as this out of the newspaper or pets for sale boards. Don't even think about a cross breed. You need the information and guidance to keep both you, your boyfriend and the dog out of trouble.
The people in this breed club will be able to give you the information you will need before considering purchasing a dog like this.
Remember you are considering a breed that has a lot of controversy right now. The banned breed legislations are spreading across US and Canada for Pitbulls and most mastiff breeds including Rottweilers. The attacks and deaths that these breeds have started to become known for isn't going to be ignored. It isn't the dogs fault, it is poor management on the part of the owners.
You need to learn aobut the breed, how to manage the dog including training, proper escape proof housing, essentially keeping the dog for 12+ years in a situation where it will never be able to harm any person or other animal. A responsible breeder will tell you also if you are in a situation that would be a good home for the dog. IF they don't feel you are ready, heed their advice and wait or change your situation to meet the requirements.
Personally, I would never recommend a breed such as this housed with a small dog or small children without utter supervision at all times and a safe separate housing for the pit bull or mastiff breed.
A mistake only takes seconds for the outcome that can be tragic and have serious repercussions. You always have to remember these are dogs bred for certain purposes. Dogs react out of instinct most of the time and that can be a fraction of a second.