Sounds like you're in the middle of a student project?
There are three groups of data here, available from three sources...
(1) Operating limitations, these should be contained within the Type Certificate Data Sheet (TCDS) for the type. These can be searched for, and downloaded, from the FAA and UK-CAA websites.
(2) Operating speeds, these are likely to be found in an operators manual for the type. There may be a generic one available from Boeing, but more likely you need to grovel to an airline ops department and see if they'll let you go and read theirs.
(2) Stability characteristics such as phugoid and spiral mode characteristics. These are not generally public knowledge - for the simple reason that most operators have no interest in the subject. Your only real chance is flight-test aircrew who have worked on the design and certification of the type. You *might* find somebody on the flight test forum, and it would be worth asking; the information will definitely exist within the company FT department, but they may not be all that keen in talking to you. If you ARE a test pilot (and I'm incorrect in my guess that you're a student) then you should also be a member of SETP, and you might be able to find a paper on the subject via the online SETP papers database.
G