Newspapers and magazines often classify their articles under names like politics, finance, business, sports, science, technology, health, etc. Collections of articles like that under specific classes may form the best corpora for your purpose. Tweets tend to be rooted in our times, deal most of the time with current affairs, and are often triggered by news reports.
This paper recently introduced an unbiased general purpose dataset for sentiment analysis of Tweets which is publicly available: https://www.researchgate.net/publication/282148966_Sentiment_Uncertainty_and_Spam_in_Twitter_Streams_and_Its_Implications_for_General_Purpose_Realtime_Sentiment_Analysis
Link to the dataset: http://project2.cs.uos.de/TweeDOS/
Conference Paper Sentiment Uncertainty and Spam in Twitter Streams and Its Im...
You will find many lexicons and usefull corpora for English sentiment classification.
Personally I used the EmoLex and it helped me to improve my accuaracy, but you can use it along with the others: The Hashtag Emotion lexicon and Corpus, Yelp and Amazon Sentiment Lexicons...