There are several sellers in eBay from the EU, mainly from UK and Germany, selling key stems, keycaps and springs. Ask them about a cable. It's likely they'll have them. In the past, I've got several pieces which weren't advertised that way. There is people in second hand shopping mobile apps selling parts as well. I don't know which one operates in Poland or even if there is any, but probably there are several in Germany, whose shipping rates are ridiculously cheap.
Another option would be buying from US through a proxy (Shipito and the likes). If the item is not bulky, you can use the cheapest shipping option, which costs about 12€/14$. You'll have to add that to the price of the item + shipping within the US, but usually it is cheaper than a direct US-EU shipping.
Third option would be buying a controller with integrated USB and Bluetooth from the german guy who is a regular here. They are not cheap, but you get top stuff you can reuse later on another board. That'll require removing the ribbon cables from the stock controller and slide them into the new one, but that's easy and a matter of seconds.
Finally, you can simply buy another Model M (with the cable). It's never bad to have another unit.
