I have never seen them in the shops but then I am not looking for them as I don't like them.
Perhaps you can try the bigger hypermarkets such as Tesco, Real, Auchan but I'm not sure if they would sell them. You could also try Marks & Spencer but again it would suprise me if you could find them here.
otherwise try some Polish "markizy" (those Oreo biscuits look totally like "markizy" on the picture) from a good manufacturer (I'm not sure who makes good ones, but a higher price might be a small indicator sometimes, just don't buy those really cheap ones)
I bought it yesterday in a big shopping center near the central railway station, the name of the shop was something like "food of the world" or "kuchnia świata", I didn't really take care of the name. But at least you know you can buy it in warszawa. :-)
Try a store called Kuchnie Swiata (World Kitchen). I heard they have Nabisco Oreo cookies there sometimes, just as they do at our Krakow location in the Galeria Krakowska. They will also order things for you sometimes, if you ask nicely.
I was going to send Nabisco brand Oreo Cookies (biscuits) to a contact in Warszawa. Is this cookie sold in Poland? If it is, I will not include them with my next package.