You need a PDB structure file to extract B factors, as protein sequences do not contain that information.
Individual amino-acids don't have B factors, but you can calculate them as an average of B factors of all atoms that belong to that residue. B factors are stored in columns 61-66 of PDB files. If you don't want to count, that would be the last numerical column in the printout below (values 15.74, 15.48, etc). Averaging their sum would give you a B factor for this MET residue.
ATOM 1 N MET A 1 21.182 23.095 19.731 1.00 15.74 N
ATOM 2 CA MET A 1 21.175 22.975 18.256 1.00 15.48 C
ATOM 3 C MET A 1 22.352 22.067 17.886 1.00 14.01 C
ATOM 4 O MET A 1 23.494 22.373 18.208 1.00 15.09 O
ATOM 5 CB MET A 1 21.350 24.356 17.631 1.00 17.44 C
ATOM 6 CG MET A 1 20.916 24.451 16.185 1.00 22.09 C
ATOM 7 SD MET A 1 21.223 26.088 15.485 1.00 26.05 S
ATOM 8 CE MET A 1 19.716 26.929 15.949 1.00 27.39 C
This is what I want! Thanks!