Trykotowe prześcieradło - where to buy this in the USA?

When i went to Poland 5 years ago, my mom bought this type of mattress cover, Trykotowe prześcieradło. It is not like a normal mattress cover, it's very soft & warm. She is wants to buy new ones but we can't seem to find any here in the USA and we aren't planning a trip to Poland any time soon. Anyone know of any places that might sell Trykotowe prześcieradło in the USA or online?

I think, any bedding shops would have that...I'm not sure.
I like 'cora' or ( 'kora'?) bedding sheet sets, including comforter cover and I usually get one set while visiting Poland.