At this moment the big brand stores have the same assortment, as any other place, so you won't find anything unique, but you can ask your family about small toy stores away from big shopping centres, as those sometimes have G3 ponies.
As for big toy stores, the biggest brand is probably Smyk, and the only interesting thing, that I've recently seen in Smyk in my city was the rainbow power fashion style Fluttershy. Other than that all the usual stuff. We also have Toys 'r Us, and thus all the TRU exclusives.
I don't live in Gdansk, so I can't tell you much about flea markets, I know there is one in Poznan, where I live
. Chance for finding ponies on a flea market is another thing though... if you visited it daily for several months, you would have some chance, but if you hope to find something in one or two visits, you would have to be very lucky. Still it most likely will be G3.
Polish auction services can be good place to hunt for those overpriced G4 euro exclusives, such us Star Swirl or Skywishes. If they appear, they sell for less than half of their Ebay price.
If you're interested, you can look for licensed items, like books, magazines, clothes etc. These can be found mostly in supermarkets and general stores.
Generally, Poland is not any special place for ponies. We got G1 relatively late and not many people seen them as collectible items, so only a few survived and mostly in bad condition. G2 also wasn't very popular, at least I didn't see many G2 ponies. G3 was quite popular and as for G4 we tend to get all the euro exclusives (including elusive ones, like Ploomette + Twilight set). That's all I think