Prison guard unions are public safety unions. They aren't lobbying to get politicians to choose their prison service, their company. They're unions of government workers. The competitor would be private prisons.

> Is there is any evidence that they are lobbying for more

Prison guard unions regularly lobby against marijuana legalization.

https://www.opensecrets.org/news/issues/marijuana/

They lobby for prison expansion, more prisons.

http://www.huffingtonpost.com/2013/09/09/california-prison-g...

They lobby against parole.

http://reason.com/blog/2012/08/22/what-does-it-mean-that-pub...

Obviously most of this is at the state level. I live in CA and know more about CA. We have private prisons, just fewer.

https://www.privateprisonnews.org/state/CA/