Bearings for the pre-1990 Cobra's are unsealed taper bearings. For the 13", 4-lug wheel, replacement bearings are available at auto parts stores like Autozone.
Large, inner bearing: Timken "Set 8" L45449 - L45410
Seal for this bearing: Timken 224015
Small, outer bearing: Timken "Set 1" LM11710 - LM11449
Wheel drums are available from Eastern Sailplanes for $125.
After recent misadventures with a disintegrating bearing en route to Florida I have added bearings and grease to the spares/tools in the trailer.