Bizarrely enough, to legally drive in Sri Lanka you need a driver's licence. An International Driving Permit (IDP) is a small booklet which translates your driving licence, so you can show people all over the world that someone, somewhere deemed you capable of driving on public roads.
Getting the correct licences is entirely your responsibility, but we do highly recommend getting an IDP stamped in category 'A' to allow you to drive a 3-wheeled light vehicle.
The requirements to get the elusive category 'A' stamped vary in different issuing countries. The situations are so different in each country, year on year that you may need to investigate further into this.
General: One thing to realise is that an International Driving Permit (IDP) is not a stand-alone document, it is a translation of your own licence. It's issued by the same organisation that issues drivers licences, and its purpose is to make your current drivers licence valid in other countries. Therefore, to get an IDP with a Category 'A' Stamp to drive a rickshaw in Sri Lanka, you need to have whatever licence from your country you need to drive a rickshaw in your own country. For example, if a Rickshaw is classed as a motorcycle in your own country, then you'll need a full motorcycle licence to drive a rickshaw in Sri Lanka.
We highly advise getting an IDP with a Category 'A' stamp, as you may void your insurance by driving a rickshaw without it.
Make sure you get your IDP from an automotive authority, we have recently heard of a few people getting them from slightly dodgy websites and the permits aren't internationally recognised.