Any on-line pharmacy that :
1. Requires a prescription from a licenced doctor.
2. Is associated with a physical store near your home so that if you have questions or issues, you can just drop in at your convenience.
3. Is licenced in your state.
4. Employs licenced Pharmacists and certified Technicians who know what they are doing.
5. Provides medication only from quality, trusted sources, so that you don't get a counterfeit drug.
6. Does not dispense outdated drugs.
... and I am just getting started here.
Seriously, if you live in the US, all the usual retail or mail order pharmacies are usually good to go.
Be careful of overseas "pharmacies". They may just be a warehouse with an off site mail box.