Hi @j0rz ,
Help but no help.
Here is a link to a page from the service manual showing the parts list for a Bose Soundtouch Portable.
Item #11 shows the part numbers for both the available colours of keypads.
KEYPAD ASSEMBLY, GREY 355698-0030
KEYPAD ASSEMBLY, BLK 355698-0010
Unfortunately a search using the part numbers has yielded no results.
You could try contacting Bose Spare parts in your location (maybe you've already done this with no luck) and giving them the part number to help them find the part.
Alternatively, can you "open" the keypad at all? If it is like a lot of keypads the contacts on the pcb are just dirty or the electrical conducting material on the underside of the button has been worn away. If it is just one or two of the most popular buttons that aren't working you may be able to fix it by cleaning the pcb contacts with 91%+ IPA (Isopropyl Alcohol - available at most pharmacies) or if that doesn't work and the conductive material has worn away you could try coating the pad on the underside of the button with a smear of conductive glue (see Ebay)