Markus,
These people may help you?
Our product range - Deep groove ball bearings - Deep groove ball bearings 62.. · SBN Wälzlager Gmbh & Co. KG, Bearings, Miniature ball bearings, Deep groove ball bearings, Self-aligning ball bearings, Thin section bearings
or :
Ball Bearing Angular Contact S 6204 E TA | GMN Bearings
Viton sealed bearings(remove 1 seal) seem rare.
An alternative - to reduce the consequences of the Buna inner seal from heat degradation may be to use a steel shielded bearing -6204 Z ? Yes, I know there is a Buna seal inside the metal shield of the ZZ bearing but it would be a little better than just the common 6204 -2RS hybrid bearing?