i know for me, i already have most of the supplies cause i've been an avid "stamper" for the last 10 years, and a lot of us are crafty like that and have the stuff. but i always get my cardstock at paperzone, a local store, they sell by the sheet and in packages, 80 lb, 100 lb, etc in MANY colors. you can go online to Paper Zone / Inspire / Design / Create to buy the packages only. over the years i've bought my supplies at paperzone, michaels, and other craft stores, as well as places like joann fabric.
also, because i've bought over the years, it didn't add up all at once for me, but i guess that's advice too late for you