Hi,
A bit late to the party on this thread, but I have renovated quite a few Sennheiser HD540 and HD250 headphones. The HD250 also has additional foam (and felt) hidden inside the drivers that needs to be replaced. Often the cushions and flexible part of the head-band need replacing as well.
Here are a few of my tips:
I stopped trying to find foam discs of the correct diameter & thickness for all these replacements. Too much hassle and too expensive (plus some are odd-shaped). In the end I looked around for suitable bulk sheets of foam, after all Sennheiser will also cut these things out of large rolls of foam they source from somewhere.
Then I found this foam on Amazon:
PC Computer Fan Dust Proof Filter Foam Sheet
or:
https://www.amazon.co.uk/gp/product/B01F8YZIMW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It is marketed as computer fan dust proof filter foam and comes in 100cm x 100cm square sheets of black foam, so you can cut out as many big, small & odd-shaped pieces of foam as you need. I held this foam up against all the different pieces of original foam in the HD540 & HD250. Honestly, to me it looks and feels the same. Same thickness (about 2.5 to 3mm), same density, same colour, same "sponginess" etc. So far (3 years and counting) this foam has worked extremely well in all the places where I needed to replace the foam in the HD540 / HD250 headphones.
For the felt I use a pack of large un-treated felt cleaning cloths I picked up from the local hardware store (Robert Dyas). It comes in assorted garish colours, but fortunately also contains some neutral blue-greyish ones. Again, this felt feels very similar to the original stuff in the Sennheisers, feels like the same acoustic density, same thickness. Being mineral felt it does tend to dull hobby knives quickly when cutting: have some spare blades ready.
For replacement of the flexible pleather part of the head-band (some are very deteriorated) I cut the same shapes out of both a piece of 0.5mm thick black polystyrene modelling sheet (incl. the location tabs) and a piece of cloth-backed black pleather furniture upholstery (excl, the location tabs), and stick the two together using double sided floor tape designed for use on vinyl flooring (you need tape resistant to migration of the plasticisers used in pleather & vinyl furniture upholstery, something I learnt from working with pleather and vinyl in camera repairs whilst trying to avoid contact glues which are very messy).
Replacement cushions I get from China. I spent ages trying to find ones that have an acoustically acceptable density of memory foam and thickness of pleather (as I have gotten older I find I now need easy-clean pleather, velours cushions have become problematic for reasons of hygiene.) These cushions so far have had the least impact on sound compared to the originals and I find them to work equally well on the HD540 and HD250. For me they are preferable to the pleather cushions Sennheiser still has available for the HD430, which also fit the HD540 and HD250. The orinal seller on eBay I can no longer find, but they seem to have changed shop-front and these appear to be exactly the same (albeit that they have now doubled the price, I used to get them for £4 per pair and fortunately have quite a stock!):
Replacement Ear Pad Cushions
Most other ones I have tried have too high a density of memory foam which emphasises the bass far too much. To be honest, memory foam seems unavoidable these days but for these older Sennheisers in general it is not such a good foam to use. IMO the original open-cell polyurethane foam is acoustically better for these old cans.
