For Federal judgments, the interest rate on judgments is determined by 28 USC §1961 for Civil cases, 40 USC 258a for Condemnation cases or 18 USC 3612 for Criminal cases. Information on calculating the interest due under these sections is posted on the Post Judgment Interest Rates page of the Federal Judiciary web site.
Treasury Rates: All three USC sections rely on the interest rates paid on U.S. Treasury securities. Formerly, the key figure was the interest rate paid on the 52-week Treasury bill. Starting December 28, 2000, the key figure became the "weekly average 1-year constant maturity Treasury yield" (P.L. 106-554).
The current rate is posted by the U.S. District Court for the Northern District of Texas or you can look it up in the Federal Reserve's latest H.15 Release.
Historical rates are posted back to 1982 by the Northern District of Texas and the Federal Reserve. Historical rates are also published in West's Federal Civil Judicial Procedure and Rules, following 28 USC 1961 . Alternatively, you're supposed to be able to get the historical interest rates by calling the clerk at any U.S. district court.
State Rates: State post-judgment interest rates are generally determined by a section of the state's statutory code. For example, New York rates are prescribed by CPLR §5004 and Ohio rates are prescribed by ORC §1343.03. You can find the pre and post judgment interest rates for all 50 states at AICPA's Pre And Post Judgment Interest Analysis Matrix. If that doesn't work, check the index of the relevant state statue or search an online edition for the terms "interest" and "judgment."