Dear folks, I am new to this hobby and about to start my first dioramas project. It seems that Celluclay is popular for making dioramas earth, but my place (Hong Kong) doesn't seem to sell such material hence I am planning to order it online. Could anyone kindly introduce which particular brand/item of Celluclay for me to check out? Thank you!

"Celluclay" is the brand name for that product. It's really nothing more than pulverized recycled paper which, when mixed with water, becomes a form of papier mache. It can take several days to dry, depending on the local humidity level, works best in thin (no more than 5 mm ) layers as it will tend to crack and split if applied too thickly, and you can improve it with the addition of some white glue. Celluclay can be colored with cheap arts & crafts acrylic paint for the earth color you may want - this will be helpful if the dio base chips after drying. Celluclay has a habit of curling up around the edges during drying and leaving a gap between it and the base. Be sure to have a little mix left over to fill the gaps should they occur. Hope this is helpful. I don't know of any similar product under different names.

"DAS" is a common brand of this type of product; air dry modeling clay is what you are looking for, and there are many brands. Get looking in a shop that sells craft items rather than modeling items, the type of place that sells paints, colouring pencils, craft kits etc, especially to children. You should definitely be able to find this stuff, it's not a specialist product. Any brand of air dry clay will do. You may find you can different colours, white, grey or terracotta.

Hi
All I use is dry plaster from the local DIY store, less than £1 for a 500g box. Just mixed with water, paint to give it an initial base colour & a little white glue to strengthen it, & apply. Takes only an hour or so to dry, allowing for time to 'embed' items, footprints, tyre marks etc..... Once covered with grass, dirt, sand, foliage etc... it does the trick!

As mentioned, Celluclay is an instant papier mache that resembles compressed lint, but turns into a moldable material when mixed with water. It has lots of issues, though. It takes a very long time to dry, the water may cause a wooden base to warp if you haven't sealed it thoroughly with urethane varnish, it shrinks badly, and may lift at the edges or even pop loose entirely when dry. In damp climates, it can even become infested with weevils.
The type of plaster sold for patching walls is an alternative. In the US, it's called spackle. There are plaster/plastic mixes like Poiyfilla in the UK or Durham's Water Putty in North America. They can be mixed with water-based paints, so you don't have to worry about any white spots in your ground cover, and they adhere well to wooden bases.
Another option is epoxy putty. It was developed for plumbing uses, but many types are now sold through art supply shops for sculpting. Milliput is the best known brand in the UK, and Apoxy Sculpt is popular in North America. These don't require water. You mix equal parts from two separate bags and knead them together, and they react chemically, hardening in a couple of hours. They can be used to sculpt almost anything, but they can be used to depict ground cover. A little water on your tools will prevent the putty from sticking to them. Tamiya makes epoxy putty as well, but theirs is more like chewing gum, and is better for sculpting parts and figures than for spreading onto large areas (it's also more expensive than other brands). Knead-a-Tite in the USA is a similar product.

I used to use Celluclay back in the day of Shepard Paine and his Building Dioramas book. I had every page memorized. That's was 30+ years ago. That brings back memories.

Today, there's a bunch of newer, more practical products. You can find a lot of them at model train stores as opposed to plastic model shops.

I prefer sculpt a mold to celluclay myself. I started with celluclay but had problems like those pointed out by the other fellows, but I don't have the same problems with the sculpt a mold.

I don't have much to add to the conversation except a "Thank you" to him_15 for asking this good question. I've learned a lot here about sculpting products. I'll have to experiment with some of these myself.
What I had used so far was a mix of carved Styrofoam block shapes and Spackle drywall paste to fix joints and smooth the form.
Cheers!
-Sean H.

I've not made many bases, but have found using extruded foam for the general ground shapes then covering with cement patch gave a very stable and nicely textured surface.
Both these items were inexpensive and available at the local hardware store.