How can I ensure I am buying a real Sapphire in Ratnapura?

Never buy stones from street hawkers regardless of how good the deal seems. Go to established showrooms and always demand a certificate from a recognized laboratory like the National Gem and Jewellery Authority or GIC. A real sapphire will have natural inclusions and a specific luster that a pro can identify. If a deal for a blue sapphire seems too good to be true, it almost certainly is.