Good suggestion,
@Adirex, and I have done that.
Ordered on LineMan.
Delivered quickly: perhaps 30 minutes.
But not cheap.
Cheaper is at large hardware stores like Thai Watsadu.
Have in big bags.
Maybe inside the store, garden department.
Maybe outside the store, alongside the parking lot, independent seller.
Price reasonable, but still not cheap.
Cheapest is little plant sellers scattered around Bangkok.
Often behind a gas station or on an empty plot of land near construction site.
They come and go wherever they can rent a bit of outdoor space.
Easiest way to find is ask your nearest "Win" moto-cycle taxi -- they know where everything is.
"Win" is the moto-cycle with the bright colored vest: usually orange or lime-green, but can be other colors.
The "Win" man won't speak English, so you'll need a "friend".