The easiest way to reply is by his nickname that his friends call him:
Con-Job
In his illustrious career John "Helllblazer" Constantine had quite a few sources of income:
Performing in a punk group

Working as PI/Occult detective

Exorcist for hire

Performing magical rituals for money, going as far as putting a demon into a corpse of a child (he was forced to do that, but he wasn't above taking money for it)

Blackmailing rich people

Pretending to do magical rituals for money

Cheating when gambling

Conjuring fake money /making people believe that he has paid them


- Borrowing money and not paying back 1
- Using his friends to pay for him
Its worth to point that John doesn't care much about having money - he once spent whole year as a homeless person. His usual needs are quite basic: place to sleep, something to drink and a pack of smokes.
1. Quite a few of his friends complain that "John owes them money". He is also quite happy to make them pay for his drinks, usually even without asking them first.