If the pads have merely gone flat, i can give you a short tutorial on re-foaming them. Some people just pull out the foam and stuff 'em full of polyfill, but i put new foam in mine.
IMHO the best place to get them is to call Michael at Beyer USA. I'm not sure of any other reliable vendor. The new ones are silver and have a silky texture. They stand straight up instead of leaning in toward the driver, gives more room for your ear.
Edit: That number is 1-800-293-4463 - if you select service you'll get Michael or his assistant. Good people. Right now, Beyer USA mops the floor with AKG USA in terms of customer service.
As for mods, some people who are very sensitive to high frequencies have experimented with passive equalization circuits installed right in the earcup, but new pads generally even out the frequency response - which is changed dramatically when the driver is the correct distance from your ear. There is one head-fi member who appears to be working on a mechanical mod to correct what he perceives as a slight sibilance in his dt-990, so, watch this forum over the next few days.