Blank cards. I got some as a gift by Carta Mundy and they are handy (lower ones in the photo). But I also got some spare cardboard bits from my father, that got them in a printer when I was a kid and gave them to me to play - they are very good anyway for prototypes (upper ones in the pict).
Xerox copier. This quite efective prototype has been printed in color on paper. Then the map has been color xeroxed on cardboard. Cards are color xeroxed on cardboard with back color xeroxed on self-adhesive transparent plastic, then carefully sticked to match and then finished with black felt pen on the sides.
Most printers can handle paper up to 203g, and will accept even thicker stock. I've been told however, that using thicker paper than the printer settings allow, may damage the valves.
So this must-have item for prototypes is 200g printing paper.
Use it for printing cards and everything else, since colors usually are clearer on heavier stock, thick paper doesn't wrinkle when glued, and just to get the right feel.
Print front and back of cards on two seperate sheets and fit them together in a sleeve. I recommend this because trying to get a printer without automatic duplex to print in the excact right spot on the backside of a sheet is a pain in the ...I allways end up turning it the wrong way, missing the spot by 2mm and so on. The final card gets thicker this way too, and that's usually preferable.
Lets you mount your newly printed gameboard on anything.
Spray it on the board you want (do it outside!) in an even layer, rush indoor with it and place your prints - it dries quite fast, but don't worry, there is time enough if you have everything ready. Straighten and press the print down with a clean rag.
Don't cut the board or the print before you mount it - it gets much better if you cut it afterwards.
There are different types of spray mount - some lets you peel of the print later ... if you can see a use for that