Professionals collect and generate weak password lists over time and often combine them into one large wordlist. Lists are generated based on their experience and what they see in pentesting engagements. These lists may also contain leaked passwords that have been published publically. Here are some of the common weak passwords lists :

• https://wiki.skullsecurity.org/index.php?title=Passwords - This includes the most well-known collections of passwords.
• SecLists - A huge collection of all kinds of lists, not only for password cracking.