A reliable one that is up to date? I'd be surprised if you do.
Data from Central Statistical Office of Poland for october 2010 (I think they publish that each 2 years, so data for 2012 will be available in this year):
stat.gov.pl/gus/5840_7257_PLK_HTML.htm
Half of the employees in the national economy was paid a total gross 2906.78 zł (median = fifth decile = central wage).
According to [wynagrodzenia.pl/kalkulator_oblicz.php] 2906 is equal to 2090.90 net.
So taking into consideration that average monthly salary in the whole year 2012 was 3521,67 zł
- nearly the same as in October 2010 3543,50 zł. I assume that median salary in the year 2012 is also around 2100 PLN
That's why I usually write that median salary in Poland is around 2000 PLN - because I haven't seen exact numbers for the year 2012 - GUS prefers to announce average, but some data are available and I think the space for mistake is not so big.