Try stationery store or greeting card store (like Hallmark cards). They may sell cardstock to people who are willing to make their own greeting cards. I bought mine from Staples (generic brand) and WalMart.
I use Georgia Pacific Card Stock from Walmart. If you have a Walmart in your area, they should carry it or something similar. Basically all you need to look for is 110 lbs. weight for any card stock paper brand.