Heh - I can offer even less help. I bought mine new
Your best bet would be your local second-hand store.
As for MD->MCD conversions, whadaymean none?
I've got at least a few right here:
Ecco The Dolphin
Ecco - The Tides Of Time
Another World/Out Of This World (Included on the Heart of the Alien CD)
Sega Classics Arcade Collection (Golden Axe, Streets of Rage, Super Monaco GP, Revenge of Shinobi, Colums)
Earthworm Jim (Special Edition)
I'm sure there are more...