Hello, I am very new to collecting. I was hoping someone could point me in the right direction. I am wondering if there are any books or websites that give the population numbers for error and variation cards. For example, I have a 1991 Pro Set #306c Chuck Knox with the NFLPA logo in cyan. How do I research how many of this variation were produced and its relative interest to Pro Set Collectors? When EBay and Google do not produce any information, what are some other avenues to glean population information? Any advice would be appreciated! Thank you.