I am not sure about other countries, but quality yogurt is very expensive in Thailand, so I sat out to see if I could make it myself. After some failures, I finally got it and oh boy is it good (but unfortunately I now understand why high quality yougurt is expensive though home made still only 33-25% of store bought)
50g store bought yogurt with bacteria
1 cup strawberry (I used frozen)
4 tsbp sugar
Pour 2L milk in a double boiler.
Heat to 85 degrees celcius.
Turn off heat
Let the milk heat for about 30 minutes until 55 degrees celcius.
Add the 50g store bought yogurt and stir with a spoon.
Sift the thickened milk sieve.
Leave the milk about 12 hours in room temperature
Sift the yogurt using a white cloth
Leave the siftet yogurt for 2-3 yogurts depending on how tangie you want it.
Beat the yogurt with an egg beater.
Add the 4 tsbp of sugar and 1 cup of strawberry again with the egg beater.
Pour the yogurt in a glass and store in the refrigerator for 10 hours.
The yogurt can now be eaten or kept for about 4 days in the refridgerator.