It would be nice to mention, which country your MacBook is from, what kind of plug it has.
Because it will be definitely cheaper to buy just a plug converter than to buy a new power adapter. Just be careful with such an adapter, most popular versions are quite dangerous used uncarefully (for example trying to plug a plug into it without looking, having it already plugged into a wall outlet, you may get electrocuted).
You need something like that:
If your laptop is from a country where the mains voltage is not 230 V or anything like that, but something similar to 110 V, ensure if your power adapter accepts also 230 V voltage (they usually do, accepting a range of voltages from 110 to 240 V, but verify it, I am not 100% sure how it looks like in case of MacBook).
In shops like Saturn they usually have such plug converters, you may check just in another one. Another popular shop similar to Saturn and Media Markt, often present in shopping malls, is RTV Euro AGD - it's a Polish chain.
You may have a look in these stores:
- those are stores selling mostly electronic components, like resistors, capacitors, integrated circuits etc., but they are likely to have also things like plug adapters.
Or, maybe even better, in these: google.de/search?q=sklep+elektroniczny+krak%C3%B3w
- these are stores selling electrical stuff, like wiring, sockets, fuses, fuse boxes, light bulbs of different sorts - so they should also have plug adapters.
By the way...
All the previous places i've stayed have had the "without the metal stick" versions available too.
Do you mean, you have an adapter to which you can plug your laptop, but it fits only to those Polish sockets which don't have this earth pin?
Not to such sockets:
Socket with an earth pin
but only to such sockets:
Then you can just buy an extension cord which has sockets without the earth pin on it.