Oops! Didn’t see your question. Beyer sells replacement earpads. But you can probably get them for less from their service/repair company Consolidated Sound Services. (I just ordered three new pairs from them, and received them in the mail today.) They also sell refurbished Beyer headphones at a discount.
Beyer recommends heating the plastic lips of the earpads with a hair dryer so they’ll stretch more easily when you put the pads on. I always seem to have trouble putting pads on my DT770’s though. So I cut a little notch in the edge of the cups (like on my old AKG K553), to make it easier to just slide the pads on by turning them.
Before putting them on, I also reinforce the seam on the inside of the earpads by sewing it with some sturdy thread. This helps prevent it from splitting apart there.
If you order pads, be sure they include the circular foam filters that cover the drivers as well. Those also need to be replaced periodically. Be a little careful when you remove them because the plastic ring that holds the filter in place also holds the driver in place.
There is also a general discussion of Beyerdynamic headphones here where you can post questions.