The Thai produced version of Sildenafil is called Sinagra and can be bought in most pharmacies, this is a good alternative and works just fine, tablets cost 400 for a pack of 4 100mg tablets. Don’t pay more than this. Most pharmacies if not all will sell these and little reason to be fake as so common, but of course always possible.
I got some in pharmacie along from LK metro opposite Areca Lodge and can say they appeared to be genuine, sealed pack that looked correct and worked just fine.
Pharmacies will sell viagra but about 3 times more expensive, also kamagra which is cheaper, your choice