I like to read both the Economist and Scientific American. Many of the articles in these magazines are written in a similar style to the RC passages on the LSAT. Some of the RC passages actually have come from older articles from the Economist and Scientific American. Scientific American is especially helpful if you don't have a science background.
Also, the NY Times has something called "Room for Debate," where about 4-6 people weigh in on an issue from different angles. This is good practice for the comparative reading passages
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