I imagine you'll want to arrive in Poland well prepared, but if you don't manage to find one in the US, those power cables are pretty ubiquitous (and probably a lot cheaper) in Europe. Many household appliances and desktop PCs use them, so they're found in electronics and DIY stores and even supermarkets all over the place.
More to the point, most analog amps use them as well, so in a pinch you could probably borrow one from another band. Or maybe ask the organizer in Poland beforehand if they can keep one handy for you.
I'm assuming the voltage should be fine as I expect its the same transformer for all countries.
In the UK its 240v.
Therefore I assume all you need is the right plug - as its a euro connector in, this should be easy to source..
Sorry lots of assumptions in there!!
Watch the pinhole in the right version. Most plugs do have it nowadays but there's still some that don't. 
Make sure yours has it.
The side that plugs into the Kemper is called a C13 connector. The side that goes into the mains is a standard European ('Schuko' type) plug.
Here's a seller that seems to be based in the US:
https://www.cablesandkits.com/power/cords/in…ab-ace/pro-266/
(I have no idea if that price is good. Googling "C13" and "Schuko" should get you more results.)
Good luck on the Polish tour! 
