I couldn’t get the medicine I needed, so I'll have to think about some kind of substitute.
I couldn't find the medicine I wanted at any pharmacy, so I'll have to consider some other kind of medicine.
この場合、次のような言い方ができますよ。
ーI couldn’t get the medicine I needed, so I'll have to think about some kind of substitute.
「必要な薬が手に入らなかったので、何らかの代替薬を考えないと」
substitute で「代替品」となり、ここでは代替薬を意味します。
ーI couldn't find the medicine I wanted at any pharmacy, so I'll have to consider some other kind of medicine.
「どこの薬局でも欲しい薬が見つからなかったので、別の薬を考えないと」
pharmacy で「薬局」
ご参考まで！
I was unable to get the medicine I need, so I will think of an alternative medication to use.
"薬が手に入らなかったので、代替薬を考えて使用します"
- I was unable to get the medicine I need, so I will think of an alternative medication to use.
- I couldn't get my hands on the medication so I will try to find an alternative to use.
- I will have to try and find an alternative as I was unable to find the medication I need.
カナダ 
アメリカ合衆国 
