It depends on how much layering ON the surface you plan to do. If you are going to use the pens for simple one pen blends then any smooth finish card stock or paper should do. 

It will surprise you as it even works on photocopy paper since you are only laying down a one pen blend. 

If you plan on laying down multiple layers of color on the card or paper and really working the blend on the surface then I have found working with card stock and paper designed for alcohol pens is best.

We really like Bee Paper

and Canson paper 

for card stock you can try xpress it card stock 

They also work great on vellum! 

We do have a digital download that you can print out and test colors on different papers - just to go to!free-downloads/c1e99