Getting good data is always a little difficult. A lot of the time it is viewed as valuable and so it is either offered for a price or not offered at all. You can find some good data through public APIs though. RottenTomatoes has an API available to developers but there is an application process.
There is also a huge amount of data available from the Stack Exchange API. What you get depends on the content of the site you are interested in and you may not get a huge firehose feed for every query but it’s worth a look.