Hi IbnSaed
You probably would not be able to get soil online, but you local Bonsai Nursery should be able to help though with Soil components. It depends mostly on what you have available locally
Soils should do several things.
be well draining and open. - The particles in the soil shoud be no smaller about 1mm in Size, and for large tress can be up to about 7 mm. It should not be just tiny grains of soil or dirt. Most of my trees I can carry in one hand have soil particles about 1 - 4 mm
the soil should hold enough moisture or the tree to use. Sounds obvious, but having a pot full of gravel is no good as does not absorb water
I use a mix of Diatomite and Zeolite, with no other components, and others use commercial organic potting mixes and other use anything and everything in between
A good mix some use is 1/3 Decomposed crushed granite, 1/3 Sifted Pine Bark, 1/3, peat moss. Depending on the environment and how fast it dries out you can vary the mix to have more Pine bark to make it lighter and more airy, but it will dry faster
If you spend all day around the tree, then a fast drying mix is great but yo will have to water several times a day. Us the work away from home this is not so good
Some people use volcanic rocks if they are available Pumice etc, instead of pine or gravel.
Have a look around the local nurseries, and Garden centres to see what you can get and then take a photo of them and post here so we can see
Also if you can describe the environment you will be keeping trees can help too. How much sun? water? is it Windy? is the area a nice Moist and humid courtyard? or a Brick wall?
Ken