Live365 allows you to decrease your payments by running ads and so does abovecast.
As for the big ad networks: You probably won't get in there at all unless you have a huge listenership.
There are a few services offering ad replacement and management and have their own network of advertisers. None of them offer an easy signup process, though. They want you to talk to a human first and only then will they tell you if your station is big enough and in the target audience at all.
Here are a few solution where I'm not sure if they actually only offer the Ad insertion/replacement software and management backend or if they actually have their own ad network.https://www.mainstreamnetwork.com/strea ... insertion/https://www.tibus.com/cdn-streaming-sol ... placement/
And then there's outlets where I'm not sure if they are even still in business:http://www.exaget.com/promo/advertisers-broadcasters/
The main company Exaget hasn't really been active since June 2017 and their ad insertion/network brand utuneme has vanished in 2015 without notice.
The Exaget website still exists and refers to UTuneMe, but the Utuneme.com website has since been taken by a scammer.