There are many sources to get sentiment analysis dataset:
- huge ngrams dataset from google storage.googleapis.com/books/ngrams/books/datasetsv2.html
- http://www.sananalytics.com/lab/twitter-sentiment/
- http://inclass.kaggle.com/c/si650winter11/data
- http://nlp.stanford.edu/sentiment/treebank.html
- or you can look into this global ML dataset repository: https://archive.ics.uci.edu/ml
Anyway, it does not mean it will help you to get a better accuracy for your current dataset because the corpus might be very different from your dataset. Apart from reducing the testing percentage vs training, you could: test other classifiers or fine tune all hyperparameters using semi-automated wrapper like CVParameterSelection or GridSearch, or even auto-weka if it fits.
It is quite rare to use 50/50, 80/20 is quite a commonly occurring ratio. A better practice is to use: 60% for training, 20% for cross validation, 20% for testing.