Buy a culture of Aspergillus niger. You can purchase it online from the ATCC, a non-profit biological resource center, Aspergillus niger is a fairly widespread fungus that can be found in many soils and some other environments.
Add sugar--any kind will do---to the culture of Aspergillus niger. The fungus will recycle the sugar into citric acid and carbon dioxide.
Filter and extract the citric acid from Aspergillus niger by adding calcium hydroxide, which will react with the acid to form a salt (calcium citrate), which will precipitate.
The chemical reaction is:
3Ca(OH)2 + 2C3H4[COOH]3OH -> Ca3(C6H5O7)2 + 3H2.
Scoop up the salt and add sulfuric acid to it. Again a crystallized precipitate (calcium sulfate) will form. But this time, you do not need it, but need to go for the liquid. This is citric acid. Here's the reaction:
3H2SO4 + Ca3(C6H5O7)2 -> 2C3H4[COOH]3OH + 3CaS04.