Try Natural Aquarium in SS19 Subang Jaya for plants. They used to carry a very comprehensive stock of plants, but less so these days- it seems that aquascaping is not so popular anymore. There is still an extensive selection of slower growing ferns and moss in their overgrown tanks. This shop also carries a huge variety of rocks and driftwood. Be careful not to gauge your eyes out when sorting through them on the shelves.

Aquascape Paradise is a relatively new shop in Section 13, Shah Alam. They carry a lot of plants and almost no fishes. The major problem with this shop is that staff don't seem to know their own stock. Many plants have melted away in their tanks, and they have sold me wrong plants a few times because their labels don't match what's being displayed.

If you like to look at fishes, make a trip to Aquatics International in Kampung Subang, near the Subang Airport. Look it up on google maps.
It's partly an ornamental fish farm and partly a biggest fish shop in town.

For a slightly smaller 'biggest' fish shop, try Xian Leng / Xian Long in Section 23 Shah Alam. Don't use google maps for this one- it still points to their old location. This shop usually have some common aquatic plants in stock.

By the way, all these shops can be reached from cyberjaya via a highway towards shah alam and subang jaya (ELITE?). The furthest one is Aquatics International, but it's worth a special trip.

Have fun!