I managed to hire a board down in Ahanghama when I was there for a week in January - around 20 boards for hire - feel free to PM me for info.
We didn't make it over to Arugam Bay given the time of year, but I've been following lots of similar threads as I fancy making another trip out to Sri Lanka in the future.
My initial impressions are Arugam is a lot less developed than whats happening around Ahanghama/Midigama so would be surprised if you can land on any rentals.
I've read very conflicting info on Arugam Bay, with some things I've read saying sups are banned