Voice123.com has an affiliate program. Depending on the size of the budget involved and the number of jobs you send their way, you could certainly pick up a few dollars through this program. Details are here.
In the interests of full disclosure, you should know that Voice123.com is one of the two Internet casting services with which I have a professional membership, . (The other service is InteractiveVoices.com, soon to be known as Voices.com.)