Start attending some local bike shows, and if possible or allowed, set up a booth with some of your designs displayed and pass out business cards or flyers. Wearing your own helmets when going out helps some too, but not as much as I've found.  You could also put your designs on websites like Etsy and Ebay so customers can contact you to do custom helmets and other items. Putting the word out on social media is a must if you want to expand your customer base.
You could possibly partner with a local vinyl wrap company to do the art work for them.