I used to live in China as well, and the difficulty in finding certain items that I took for granted in the states was very annoying!
As far as hydroponic
nutrients go, maybe I can help.
Plants need 6 macronutrients. These are Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S). The "NPK" that you see on the fertilizer bags stands for "Nitrogen - Phosphorus - Potassium". Even if it were water-solubal, you'd probably still need to add calcium, magnesium, and sulpher.
In addition to the above, plants also need a small amount of Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo) and Chlorine (Cl).
The simplest home-made hydroponic
nutrient solution that I've heard of is a compost tea made out of composted chicken manure. I don't know about the availability of composted chicken manure in China & Hong Kong, but I do know that chickens are popular there, so it's possible. If you make a chicken compost tea, you might want to add nitrogen and phosphorus. Some plants, such as tomatoes and strawberries, need a lot of nitrogen, and phosphorus is important for any flowering/fruiting plant.
On a side note; most soil fertilizers use Urea and Sulfate of Ammonia as sources of nitrogen. These do not work well in hydroponics because they need bacteria to break them down before plants can absorb them. Instead, you should use Ammonium Sulfate or Sodium Nitrate.
The recipes that I've found for home-made solutions are:
Potassium Nitrate 9 oz (255 grams) - Also called Saltpeter
Calcium Sulphate 7 oz (198 grams) - Plaster of Paris
Magnesium Sulphate 6 oz (170 grams) - Epsom Salts
Monocalcium Phosphate 4 oz (113 grams)
Ammonium Sulfate 1.5 oz (43 grams) - Principle source of Nitrogen in recipe
Iron Sulphate - Use a pinch only; maybe 1/2 t
Grind each chemical up and mix together well. Use 1 teaspoon per gallon of water.
Another recipe is:
Sodium Nitrate 12.5 oz (355 grams) - Nitrogen
Potassium Sulphate 4 oz (113 grams) - Potassium, Sulphur
Super Phosphate 5 oz (142 grams) - Phosphate, Calcium
Magnesium Sulphate 3.5 oz (100 grams) - Magnesium, Sulphur
Iron Sulphate – 1 pinch (2 grams) - Iron
Once again, use 1 teaspoon per gallon of water.
Here is another recipe solely for trace elements:
Manganese Sulphate 1 t
Boric Acid powder 1 t
Zinc Sulphate 0.5 t
Copper Sulphate 0.5 t
These can all be found in mineral vitamins at a health store.
When needed add 0.5 tsp to 1 quart water and mix well. Add 1 liquid oz of this solution to 3 gallons of your plant nutrients solution from above. This solution will not keep past one day, so use the remainder to water any "in-soil" plants you may have around.
I have not tried any of these recipes yet, so I have no idea how well they work. If you do try one, please let me know!
Hope that helps!