Extra.Dat's are only sent out individually to clients to cover erroneous detections usually. You can download the current DAT or SuperDat here: http://www.mcafee.com/apps/downloads/security-updates/security-updates.aspx
Unfortunally I need extra.dat file. I already have one extra.dat but this file out of date. May be you can post it here?
Why do you need an extra.dat? If the one you have is old then it is most likely included in the current DAT's anyway.
You can only get an extra.dat by sending in a detection to McAfee Labs and if they find it isn't an infection, they send you an extra.dat.