Way down in the article is states: "In 1991, only 649 Alfas were sold in the USA."!?!! Is that an accurate number? I just bought a '91 & was very surprised by this count...
Polk data is widely used in the automotive aftermarket industry. RL Polk works with state governments to collect actual registration data. Many companies buy the information directly from Polk. It is also printed in an annual publication called Ward's Automotive Yearbook. You might find copies in a good business library. The 1992 Yearbook will have data for 1991 model vehicles.