Where can I find a smaller assisted living residence for my brother with Alzheimer's who has a criminal record?

Most federally subsidized assisted livings are not available for someone with a criminal record. Some have exceptions for certain kinds of records. In Florida there is a law that bans felons from certain housing and nursing homes. It's called Norma's Law after a young woman whose murderer was found living in a nursing home. Residents and their families had not been notified by the state. Perhaps you could find a private/corporate owned AL that would accept him.

What state is he in and how old is he?

You might try to find a social worker who specializes in finding living situations for the elderly. We used one for my Mom, and she was very helpful in knowing all the options and pros and cons. The one we used worked for a local firm that does this work.