Rx Pharmacy, close to the junction of Sukhumvit and Soi 4:
https://maps.app.goo.gl/9sm15365t9CGi1fc6
No drama at all with this place. Definitely no prescription needed. Just ask for Sidegra, either 50mg or 100mg, and tell them how many boxes you want (4 tablets per box).
Sidegra 100mg is 250 baht per box. They accept cash or card.