Is Sri Lanka has a zip code , If not, what should i put under the zip code?

Are you trying to fill out some online form, again? Are you trying to send money through Western Union, or something? Just skip over that "field" (that space): you don't have to put anything, in there. P.S: You must not be an American; because, U.S. citizens know that there is no such code that represents a whole country, like that.

put anything if its an online form, or use the search bar to find "Sri Lanka post code.

Zip codes are a series of number codes assigned for an area. For an example United States of America is a big country with so many cities under different states. Sometimes the same name town is repeated in 2 or in somecases 3 states. So in order to seperate the city from each state the Zip coding is introduced. But Sri Lanka for that matter does not have any Zip codes we have post office codes which is not recognised in global terms.