To answer that, I'm going to use Ebay as a gauge. I will search for any disc from the set. The more that are available, the easier it should be for me to get the missing Cubs.
1976 Carousel - missing Cardenal, Madlock, Morales, and Thornton. This is a 70-disc set. My Ebay search turned up 19 listings. That means that there are a lot more players not available than just my three Cubs. This will be tough
1976 Red Barn - Missing Madlock, Monday, Thornton. This one will be even harder than Carousel. Just 12 listings were found on Ebay for the 70 discs.
1977 Saga - Missing Cardenal. I found 30 listings, which is far more than the other disc sets, but no Jose. I'm a bit more hopeful that one will surface.
1979 Wiffle - Missing Buckner, Cardenal, Kingman, Trillo - There are 88 discs in this set and I'm missing four of the five Cubs. Ebay has 42 different listings, but none of those include my missing four. Another difficult one.
1985 Thom McAn - Missing Sandberg and Sutcliffe. This is another one that may take a while. There are 46 discs in the set and just 13 Ebay listings.
1985 Subway / Blank Back - Missing Sandberg and Sutcliffe These are blank backed discs but were given away at Subway shops. I found just seven available on Ebay.
1989 Key Food - Missing Dawson. The set has 20 discs and Ebay has five listings.
1989 Our Own Tea - Missing Dawson This set is identical to the Key Food discs with just the brand name switched. Six listing are on Ebay including one for Dawson. The price, though, is $25 and that is a bit too steep for me.
So there you have my missing 21, which will most likely be missing for a while.