Hmm, I can get a copy of my old tax returns, but this isn't really the same as showing that I don't owe any money. I don't think the IRS issues any kind of "no outstanding balance" documents.
Best contact the US consulate in Poland and they will advise you about what document to obtain.
Can someone confirm that this applies only for non-EU citizens?
I have temporary residence in Poland, am from an EU country and didn't have to supply proof about any outstanding tax liabilities I had.