How do you get credits?

http://mafiareturns.com/credits/purchase.php

there you go mate use that link if u wanna buy with real money

You can either purchase them using money you've accrued in Mafia Returns, or donate real life money to the game to support the development and sustainment of our community. 

Go look at the Credit Marketplace under Manage -> Credit Marketplace.

Or visit the marketplace to buy with ingame money.

http://mafiareturns.com/credits/marketplace.php

Either buy purchasing them by using game cash here: http://mafiareturns.com/credits/marketplace.php

Or by donating to the site with your real cash here: http://mafiareturns.com/credits/purchase.php

Alternatively, I think a few of the achievements still give out bits and pieces of credits, you can check your achievements here: http://mafiareturns.com/news/achievements.php

So the only way is to either pay real money, or buy with in game money?

Yes sir.

Some achievements will give you a tiny amount of Credits. Also, keeping an eye on the Mafia Returns Facebook page could be beneficial to you if you are the first to redeem their Credit give-aways.

With a small amounts offered through ingame achievements

If you enter the Lounge Chat, there are CONSTANT rewards there. 

Click on Chat >> Loung Chat (Rewards).

Talk in there while you're doing your petties and felonies.. and you'll see credits get listed.
You have to be quick though, as it's a matter of speed. Whoever claims the credit first gets it.

If you're active in chat, you also then get additional rewards that are sent to just you, or you and a smaller group of people to increase your chances of winning it.