21 years of maps, eh? You won't be able to get them from the MTA. They would only have the latest editions. The NY Transit Museum also does not have any old maps for sale.
You may want to try an online auction site like ebay. I saw a 1996 map available there fro $1.00 as a starting bid with a $5.00 "buy it now". (Don't forget that there will be a mailing cost on top of that).
If you know of anyone who is a subway buff/railfan, maybe they'd have some extras they would be willing to part with (for a price).
Kelvinhall subway station was created in 1896.
Ibrox subway station was created in 1896.
Cessnock subway station was created in 1896.
Cowcaddens subway station was created in 1896.
Kelvinbridge subway station was created in 1896.
Govan subway station was created in 1896.
Partick subway station was created in 1980.
Hillhead subway station was created in 1896.
the synonym for subway is train station
Shields Road subway station was created in 1896.
West Street subway station was created in 1896.
Kinning Park subway station was created in 1896.