BOL

At UniProt search for taxonomy:mammalia in UniProtKB like this:

http://www.uniprot.org/uniprot/?query=lineage%3Amammalia&sort=score

There you can download a fasta file with all mammalian sequences with isoforms like this: 

http://www.uniprot.org/uniprot/?sort=score&desc=&compress=yes&query=lineage:mammalia&fil=&format=fasta&force=yes&include=yes

or maybe just canonical reviewed sequences like this:

http://www.uniprot.org/uniprot/?sort=score&desc=&compress=yes&query=taxonomy:mammalia&fil=reviewed:yes&format=fasta&force=yes