Hey Mark, recently just became a HD250 owner, and really they are outstanding. You'll find less info on these on Head-fi (oddly, I just don't think many here know about them), but on Graham Slee's forum - both current and old 'uk' one - you'll find threads.
https://www.hifisystemcomponents.com/forum/_topic1592_page1.html
Also, search for 'Graham Slee HD250' in Google. From what I understand, a number of pads can fit, even some Beyer models. Ebay has listings for very good prices for HD250 replacement pads. You may not want the cheapest of cheap pads, but some sort of protein leather should be fine. Also Kennerton who makes the Magister, makes some high-end earpads
http://kennerton.com/. You may want to get the softest pads to improve seal.
I have yet to order pads for mine. I got a used pair that were liquidated from Radio-Canada (national broadcaster), and looks like they had them recabled and pads replaced. The pads I have work sonically well, but the foam is nowhere near as soft as my Denon D2000 pads - those are sublime.
I've owned these for about a month now, and pretty much they are here to stay, as they are the most balanced sounding headphone in my collection. They sound great with everything. A true genre master.
edit - also look at this thread
https://www.head-fi.org/f/threads/wow-sennheiser-hd-540-reference-are-so-good.670255/