present a few difficulties.
Buying printed ones a) costs money, b) adds the complication of shipping, c) probably requires a credit card, and d) results in depending on a particular vendor staying in business and not changing anything.
Using an image from the web and printing it seems stabler (you can swap copies with bittorrent if it comes to that) but runs into the problem that different printers may print it differently.
Best might be a different approach: take spore print photos on a white/black divided background with your camera set to auto-white-balance so the white half is used by the camera to adjust the image tint and correct for lighting and any camera-specific idiosyncrasies. With any luck, the same spore print will come out the same RGB colors in the resulting digital photos if taken with this technique. Then the only color cards you need to get consistent are the black and white spore print cards, and any printer can print solid black and solid white consistently. Perhaps this should be tested: photo the same spore print, with auto-WB, with several common camera makes and models and see if the results are reasonably consistent if the print was on a divided black and white card. If the results are consistent, then we can just take such photos and compare the colors. Even though our monitors might not display colors identically, if jpegs with the same color are compared on the same monitor they should still look the same.
