If you are looking at a few options, paying your listeners to help you find guests might be the best one of all of them.  Compare it to hiring a booking agent or assistant.  There is a good chance that you are not going to find an assistant that knows your industry umbrella and particularly your niche.  But, you listeners absolutely will. 

Your listeners are just like you.  And they are just like your other listeners.  So, there is a great chance that if they suggest a guest, this is a guest that the rest of your audience will enjoy hearing.  You are leveraging your platform to make it even better.  And, if your show is like mine then it is going to be difficult to mine those diamonds out of the rough.  Your listeners will have insight because they are interested in this as well, and there may be no other way to find those great guests that are just waiting to be discovered.