GRE Text Completion practice questions — where to find them?

ETS has three main books: the Official Guide to the GRE, the Official Guide to the GRE Quantitative Reasoning, and the Official Guide to GRE Verbal Reasoning. (Confirm exact current titles on ETS.org, as editions and names can change.) Official practice questions are also available through digital resources on the ETS website.

These are the gold standard for practice material because they're written by the same people who write the actual test. Third-party questions can be useful too, but always start with official material.