The D2 adapter is the same as adapters for other EOS switches (S-Series, K-Series, N-Series, G-Series, 800 Series, and so on). At least the 800 Series console cable is still in the Extreme Networks price list (08A-CON-CBL, 800 SERIES CONSOLE CABLE KIT).
If you are looking at used cables / adapters, they might be called Enterasys instead of Extreme.