The best option and also the cheapest one is Social Media. You can create a page or a group, or simply make a post on Instagram or Facebook to make the announcement. Another option is to share flyers, but this will take some time and it's not that efficient as the social media option. I and my band worked with mobilestagehire.co.uk on our last concert, and as soon as we announced that we're working with them, the tickets sold out. Definitely, our audience enjoyed our performance, and also our set up.