suggestions for good sunscreen (Colombo)

hello everyone,
I have been living in Colombo for about ten days now and I am trying to learn step-by-step, how to fit in... and I have more than few questions, so expect me to post many silly, basic questions here.

But my very first question would be, about shopping a good sunscreen in Colombo.
As I like traveling lightly, I only brought a small tube of sunscreen and I am afraid I will soon need to find replacement. I heard that I'd have to go to pharmacy for one, but could any one give me some recommendation? I am not sure how strong it should be( Protected content ) or which one is without harmful chemicals, etc.

Thanks in advance.
Really nice weather these days and wouldn't want to miss out, b/c of sunscreen penurie ;)

Doyen