I thought this deserved its own thread, as an important item for vintage enthusiasts. The balata ball is no longer made and those seeking that feel must either break out their old stock of balata or try some modern ball that comes close.
My personal notabalata ball, for softness (but not spin) is the Wilson Staff DX2 soft. (DUO soft in North America). The DX3 soft comes closer on spin, but loses some softness. (Duo urethane in NA)
Otherwise I just use the tour b series from Bridgestone.
What ball do you play with your vintage equipment?