Do you know any repository where can i find .prj files?
I am looking for local coordinate system: Warszawa 75 (Warsaw 75)
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Besides agreeing with Brad / whubber on everything they've said, I suggest a google search, an examination of your metadata, and an examination of the coordinates in ArcMap. Drop it in a fresh map, and look at the coordinates. This may give you some clues. As for the other info I've suggested, see all the comments at this link as well.
Copy and paste the name of the original shapefile in google (including the .shp extension). It may reveal nothing, but you may also be surprised whet you find out!! People underestimate google searches!!!!
There is also a site called http://epsg2prj.org/. But seems to be down at the moment.