Please note I haven't purchased anything from these places before! I'm looking for assorted stuff, so I haven't really checked the individual colours, but here's what I found.http://www.thepapermillstore.com/cardstock/
However, I did find that most of them ended up being around 30cents a sheet.http://www.paperquilling.ca/shop/
Depending on what you get, 30-45cents a sheet.http://www.cutcardstock.com/
A wide selection of stuff, anywhere from 16-45cents a sheet. They've got a lot of the bright colours, so you might find it here...
And of course your local arts/crafts stores.