Many cryptanalysis texts will provide a sample frequency table. A good text will also advise that, for best advantage, a frequency table should be derived from a corpus (collection of documents) that are representative of the text you'll be using the frequency table to help decrypt. So, if you're going to mount a brute force attack on a simple substitution cipher, it'll help to derive your frequency table from a set of works that are representative of the text that's encrypted.