I am searching a sennheiser hd 238 spare cable but can't find it.... Any suggestions?

It appears that the 238 has a cable connected to one headphone only.
If this is the case, try your existing cable.
If both ends look identical and plug into your music player easily, you could try any audio cable with
3.5mm mini plugs on each end.

I have used such to drive low powered speakers, and discount the hype about audio cables.

If you look on eBay, entering "sennheiser hd cable", a number of sellers have cables, none specifically claimed to fit.
The cheapest among the first few entries advising the cable has a 3.5mm plug at one end and a 2.5 mm plug at the other. If my first suggestion is unworkable, this will probably work.

Alternatively you could try to find one locally, to test for fit before purchase.

Check the other sellers for a look at pictures and possibly details if I got it wrong at the start and your cable has 2 connectors at one end.

If all else fails, take your headphones to you local electronics supplier, get plugs that fit and get a good friend with a soldering iron to make you up a cable.