If you like to make fresh Doufu yourself, here is steps :

Ingredients 
Soybeans  100 g      Water  10 cups      Natural Bittern  4 g

Steps
1)   Remove moth-eaten & damaged soy bean, impurities as well
2)   Rinse the soy bean in clean water 2-3 times, soak then in 8-10 cup of water
3)   Soak the soy bean for whole night until they expand to 3 times larger
4)   Drain out the water, then put the soaked soybean into a blender
5)   Add 2 cups of clean water, turn on the blender for 2 minutes
6)   Pouring 8 cups of water into a deep flat-bottom pot, and heating until boiled
7)   Turn into middle heat, put the grounded soybean in and boil for 15 minutes
8)   Stirring with a ladle when boiling, but NOT USE high heat
9)   Fill the “bean-milk” slowly and little by little into a cotton gauze bag, and wring it
10) If the bean-milk is too hot, wring the bag by chopsticks, until all juice is wrung out
11) Put natural bittern in small container and add 1/2 cup (100cc) water to dissolve
12) Boil the bean-milk with slow heat until 70℃, turn off fire
13) Add 1/2 of natural bittern solution and stir gently
14) Keep the temperature by 70℃, boiling the bean-milk for 5 minutes
15) Add the other 1/2 of natural bittern solution, keep stirring gently
16) Keep the temperature by 70℃, boiling the bean-milk for 15 minutes more
17) Don’t let the temperature too high or too low, or the congregation will be poor
18) When all bean-milk is congealed, water will be separated out
19) Spread a bleached cotton towel in a plastic rectangular box
20) Then pour the congealed bean-milk into the rectangular box
21) Wrap it well in the towel, then add a flat weight onto its top for 10 minutes
22) After further congeals, remove the wrapping in water
23) Wash surplus bittern away with water, beancurd done

Remark 
Step 21 – its better to use a light aluminum plate with 3-4 cup of water
You can purchase beancurd in any food-market of China-town