I have been lucky because people around me are readers and willing to buy my books. I have also been lucky enough to have met other local authors and been able to purchase their books. This is also a good reason to join a writing group, to meet these people and to get support from the group. This kind of support is what every author is looking for, because it usually means that you cared enough to spend your money. Further support is to put out a good review of the work.
If you want to fully support an author buy their books directly from them, or their publisher. I know it is cheaper to get them second hand, or through a third party, but those options mean little or no money gets to the author. Most authors do not write to make money, but the love of the story doesn’t pay the bills associated with writing it down. So, if you know a local author who needs the support, buy a copy of them book from them. You get some entertainment from reading it and they get the support. Also they will likely sign it for you.