Any paint store has an additive packet that you can mix into a gallon of paint for mildew resistance. Look for a 100% acrylic gloss or semigloss at any good paint store.
The best mildew preventive is ventilation. If you are sprucing up your bathroom, now might be a good time to address that. A small bathroom should have a minumum 100 cfm fan. The builders original might be half of that. For a modest $$investment, you can get plenty of CFM and also quieter. Look for maximum 1 sone.