The site listed above seems a little pricy. I usually order blank cards from Amazon for around $15 with the shipping (100 count). Of course these are standard thickness credit cards. The site above has thicker ones.
My goodness there are a ton of sites you can order inlay materials from. I use sites that deal with billiard cues as well as toys and models. Plastistrut is huge for black or white plastics in all thicknesses. Then there are acrylic dealers that sell sheets that I use also. Can get all different colors.