Without knowing much about your podcast or ultimate business goals here, I would say reaching out to the potential sponsors to at least get a feel would be a good idea. You might even consider some promotional sponsorship rate for one or two sponsors to get in early. If you have a marketing plan to show how you are going to work to drive the traffic necessary to make their sponsorship worthwhile, then they might be willing to take a chance on you early. Rewarding them for taking that risk with a slightly lower rate could be good for both of you. Good luck and enjoy!