Buy only from the Agent.
Check the official websites of the brands you are interested & find the registered service agents (not seller agents, because seller agents don't repair them with good company standards. Seller agents just sell the units) of sri lanka for that brand.
Normally Singer,Abans,Softlogic, John Keels Computers are the trusted agents, depends on the brand(like John Keels computers for Asus).
Don't buy devices from shops in Unity plaza like laptop.lk, techzone etc.
They may sell laptops with bit lower price (rs. 3000, rs.5000) than agents.
But if your device breaks or needs a repair, even within the warranty period, you'll be screwed up.
You'll have to forget the money you spent & buy a new one. I have the experience, that's why i'm telling you.
Only buy from a Registered Agent.