How to get all the users on a database with SA access?
The simplest scenario (a user explicitly granted sysadmin directly) is covered by the following query:

SELECT p.name
  FROM sys.server_principals AS p
  INNER JOIN sys.server_role_members AS m
  ON p.principal_id = m.member_principal_id
  WHERE m.role_principal_id = 3;


But then you may also have users who are members of a Windows group (which SQL Server won't know about) and that group has been granted sysadmin. To get all of those users you may need to run some queries against Active Directory (or ask someone who can).