Because of today's technology, academic research is becoming easier and more available to more people every day. Your two biggest options for Roald Dahl research are the Internet and the good old fashioned public library.
On the Internet, I have found Google Scholar to be one of the best seach engines currently available for scholarly journal article searches. If you go to www.googlescholar.com, you can try different key word searches that often produce full text articles which are available to view for free. The link below will actually take you directly to a search that was done using the keywords [Roald]+[Dahl]+[criticism]. I usually advise students to try a few different searches with different key words and allow your results to guide your next search.
In the library, you are going to want to look for Roald Dahl criticism among the non-fiction literature books. Certainly you will find most of his fictional works if you simply type [Roald Dahl] into a search engine. But if your public library is lacking in books of criticism, you might see what collegiate libraries are available for your use. Many public libraries in towns and cities boasting of major universities have reciprocity with the university library, which means you can often utilize their services for free. If you must travel outside your county for such a library, you may not be able to check anything out, but certainly you could find books and photocopy necessary information.