2 quick and simple ideas come to mind.

1. Open up iTunes, open the podcast section, open up your relevant categories and subcategories. e.g. business/business and marketing.

Contact the podcasts that are in the "New and Noteworthy" sections or in the lower half of the top 200 podcasts sections.

2. Open Twitter or Tweetdeck and use Twitter search for people using phrases like "marketing podcast" or "marketing podcast guests" etc. Try a few different types of query and see if you can use Twitter to narrow down some of the marketing podcasters that are looking for guests.

Good luck with your new book!

Scott