It should be the same way you obtained your ancestor's birth certificate that you used to prove your claim to citizenship.
When I needed my father's BC, I got a form from my local consulate where I listed the details, paid a fee (asround $50, don't remember) the consulate did the legwork and the consulate sent it to me via certified mail once they received it. Took a couple of months.

 

You can do it directly through the Polish consulate. It takes about a month as well.