Finding targets of transcription factors
If you were interested in human targets, one easy way is GeneCards, which aggregates other resources, and combines computational and (some) experimental data:

In this case the inverse problem is particularly well accessible: e.g.: scroll down to section of Genomics (e.g.: for the gene called NOTCH http://www.genecards.org/cgi-bin/carddisp.pl?gene=NOTCH1&amp;keywords=notch1 ).

If you you were interested in obtaining all genes of a given transcription factor, you could buy a license, costing around 150 USD per year, for the GeneALaCart service of the company that runs the free genecards ( https://genealacart.genecards.org ), upload a list of all human genes (which you could for instance obtain from ncbi's ftp server), and get a spreadsheet linking transcription factors to genes.