Rainco has a big shop in Dehiwala selling everything from (expensive) patio furniture and awnings to affordable camping equipment and umbrellas.
It's a big space and the ground floor is devoted to lawn furniture, hammocks and stuff like awnings. You can get big sun umbrellas starting at 13,000 and wood chairs from Rs. 7,500 up. If you're looking for that sort of thing, this shop isn't cheap, but it seems to be of good quality.
If you go upstairs they have rather affordable camping equipment, which we think might be of more interest to YAMU readers. You can get a decent tent for around Rs. 6,000 upwards, or a 20L portable toilet for Rs. 10,000. Throughout, of course, there's a wide selection of umbrellas.
Go upstairs to check out the camping equipment.