If you mean Cardstock in general, like
>>426741Said, I usually get mine from officemax
If you mean A4 sized in specific, yea, I have a hard time finding it in retail stores here in america.
But Legal sized, while not as common as standard sized, is a bit easier to find in cardstock and it'll have more then enough room to fit an A4 sized printout on it
And here's some at amazon
http://www.amazon.com/Wausau-Cardstock-Brightness-Inches-49413/dp/B001EQ667A/ref=sr_1_1?ie=UTF8&
qid=1299708210&sr=8-1