Spare parts are truly quite a problem with prams - and the more so with these affordable Polish-made prams (like the Baby Merc Zipy Classic is). The shipping cost will also be considerable - if you even find someone who sells these.
As usual with spare parts, you need to try to contact the seller you bought your pram, or any sellers/distributors of the Baby Merc brand, if there is any around you. They should be able to order spare parts from the manufacturer.
You can also try to contact the manufacturer, Baby Merc, directly.
Another thing to try is either to find used Zipy Classic stroller systems in classifieds and buy this part used.
And the last thing I'll suggest to you is to try to write to this eBay seller, sklepbabyshop30 - they do sell Baby Merc prams online and might be willing to order and sell the adapters to you.