Board rental costs about 1000LKR per day, but you should be able to negotiate down to 700LKR in low season or for renting one long-term. The general condition of boards is improving slowly with more investment. The best option is to buy and resell if you are coming for more than a month. Try the Arugam Bay Surf Community Facebook group if you want to sell.
There are some very popular scams played out with rental boards, but there are some good and honest businesses out there too. Ensure that you check the board over well and make a note of any existing repairs or holes. It is standard practice to leave a passport or driving licence as insurance but this puts you in a difficult position if the worst happens. The best advice is to agree on a price that you will pay if the board is, for example, broken in half and unusable. Some renters have charged the new price for breaking a 20 year old board!
Board repairs are extremely expensive in Sri Lanka but the guys in general do a good job. Just be careful of the Sri Lankan surfer in the line up with the dented board. He might just be there to engineer a collision with you to give his friend some repair trade!
For a listing of surf shops in Arugam Bay check out our business directory.