It is quite easy to get a travel SIM card in Sri Lanka. This can be done at the airport. It is also important to have internet access as it is hard to find internet outside CBD. Once you get a SIM card you can easily add 800 MB by paying 0.5 USD, which is valid for 3 weeks.
The best internet card would be a Dialgo SIM card. Once you add money to activate your SIM, request specifically for an Internet card according to your Internet needs. Keep an eye on the internet usage since once the data is fully used, it will start using the credit on your mobile phone, which is much expensive than having a dedicated internet card.