One of the previous posters mentioned a.net because that is more of an airplane/aviation-geek site whereas flyertalk is much of a frequent-flyer/milage-geek website.
That being said, I do lurk around a.net from time to time, but there is a lot of garbage to wade through. Same can be said for this site too at times.
Anyways, to answer your question:
DL 907 MSP-ORD on 9/16 was operated by ship #9886 - N600TR
One way to find this out is to go to flightaware.com and in the flight # box enter an aircraft registration number. I just started plugging in numbers that were close to what you said. Not all fleet types and aircraft work, but it works for the DL DC-9s.