What type of acid is found in tea?
Some of the major components of tea are polyphenols. They are fairly acidic and may dominate the pH of tea.

For some examples of the types of polyphenol that are common see this site on tea chemistry:


  The major flavanols [a specific type of polyphenolic compounds] in tea are: catechin (C), epicatechin (EC), epicatechin gallate (ECG), gallocatechin (GC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG). 


EGCC has this structure:


from which some degree of acidity is likely to result from all those phenolic OH groups.