These are a few sites i found for that, i am not an R programmer so i dont know any Weka tools and how to use them but hope this helps.

 http://www.sananalytics.com/lab/twitter-sentiment/sanders-twitter-0.2.zip  https://archive.ics.uci.edu/ml/datasets/Twitter+Data+set+for+Arabic+Sentiment+Analysis

These are a few sites I found for that. I am not an R programmer so I don't know any Weka tools and how to use them but hope this helps. You can find them here and here.