How can I find a replacement derailleur for a nice bike that is 20 years old (GT Winstream)
Good news. Shimano kept broad compatibility between their products, even between generations.
An Altus derailleur from circa 2002 may be from a 7 or 8 speed (number of gears in the rear cluster) drivetrain. You can actually use any Shimano 7, 8 or 9 speed derailleur with a 7 or 8 speed shifter. Shimano used the same derailleur actuation ratio (ratio of cable pulled to lateral derailleur movement) for all these drivetrains.
Remember you need to select a derailleur that has a sufficient total capacity (capacity required is (difference between max and min sprockets teeth) + (difference between max and min chainring teeth), and has a appropriate max and min largest sprocket size for the sprockets on your cassette.