You can buy as many as you want without prescription, they dont ask you for one, the prices for the original are high but you can buy legitimate clones for the 1/3 of the orirginal price...
I would recommend sindelafil Sandoz (viagra) 100mg , it has 4 pills 100mg each but you can brake them in quarters and have 25mg each as you need.
Total cost around 15€
Same for sindenafil but those brakes in half and you take 10mg per dose
You choose
Have a nice day