Well i suggest you going to Karu's music shop in Nugegoda (in Nawala Road adjacent to the Weli Para. You can get Daddarios for 850 if you are looking for Electric guitar strings. I saw that they had some Ernie ball acoustics aswell.
Other than that you can also check out music makers (small shop but its very decent) in Nuegegoda.
And finally Freakout in Gamsabha junction near the Highlands school.