Prepare a letter explaining your desire to become a broker and mail it to all the printers in your area. The smaller printers seem to be the most likely to work with you on this. Set up a time to meet with any printers who are interested. You won't have to explain much because almost all printers are familiar with brokering. You should be able to find printers interested in working with you, even those who already have other brokers.
Printing is a very competitive business and printers always want more work.
So, you should have no trouble setting up connections. Try to get a price list of YOUR costs drawn up by each printer. This will help you if someone needs a price quote on a job. Also, you can check the mail order publications and see what other printers are charging. Work out an arrangement where you will have flyers, circulars, ad sheets, etc, ready to be printed whenever the printer has down time.