Hi,
Natural sources of
vitamin c are citrus fruits, like amla, guava, orange ,lemon etc. Generally from fresh fruits and vegetables vitamin c can get. To build up immunity is a gradual a process, it can't say that within particular time period you can have strong immunity. Instead of supplements it's better to get vitamin c from food which is more bioavailable.
Health Tips
Maintain a healthy lifestyle with having balanced diet surely help to boost immunity power.