I don't know what to do when an Ukranian back needs a new seal.
But i cut seals for my Swedish backs myself, from an ethylene-propylene-diene monomer 'semi-closed' cell foam.
The stuff sounds fancy, but is available in sheets from hobby shops, for very little money.
They work as good, if not better than the original thingies.
And so ridiculously cheap that you would start changing seals just because of it, long before it would be necessary to even start thinking about doing so.