There is an eat way to grind this out.
First of all, you need to have advanced to chapter 2 to unlock the funfair. Before you head there, go to Hector's house and collect the worm. Then head to the funfair and go all the way to the right. Speak to the race carnie and select the worm race. Each time you win you get 5 coins and don't have to pay to enter either.
This has been the quickest way for me personally.
Edit: Skate 323 has added that winning the horse race rewards you 40 coins per win so this works out quicker.