I know this is old but there are some non-union talent that will now work out a retainer with a production company. But that will not give you the flexibility that you are looking for.
The main issue that makes what you are looking for difficult is usage. If someone set up a retainer type website, how could they cover all of the different types of usage that are required? A 30 second global TV ad can pay 5 figures for a well known brand yet a 10 hour audio book can be $3K. There are just so many parameters that make it difficult so I feel your pain.
You just want a nice easy solution with a fixed monthly budget. Let me know if you find something.
Craig’s Voice Over Website: British Voice Over.
Craig’s Blog: British Voice Over Blog.