How to apply for a Sri Lankan residence visa for a uk citizen

I would like to move to Sri Lanka for a year starting in July-not to work just for a year off work, therefore can I get residence visa once I arrive in Colombo (and enter the country with a 30 day entry visa) or do I need to apply for the residence visa from the uk? Also what documents will I need to apply? If anyone can help I would be very grateful!