HI
This is not a commercial forum.I would suggest you contact one of the puppyline representatives at
http://www.mastiffclub.com/puppyline you will find 2 experienced mastiff owners telephone numbers listed.
Doing your research is very important and if you find breeders that have stock you are interested in I would suggest a visit to the UK at a time when you can take in a couple of shows. Meet people and their dogs, at the present time with the economical situation I imagine that there is not so much breeding which could be a good thing.
Sadly there are people who advertise on the internet who are not as knowledgable as they imply. The saying "let the buyer beware" should be observed.
Both clubs hold Ch shows and the Ch shows are listed on the club website, winners photographs if available are posted.
Good luck and many happy years with our wonderful breed.
LS