One thing to do is to get involved in communities that are relevant to your niche and then find "passive" ways to promote your show within those communities. Don't join a relevant Facebook group and then just start spamming up the place with links to your show. Instead, engage in conversations. Provide value to the people there. And when it may become appropriate, you can drop a plug for your show.
Another thing would be to go to conventions/meetups that are relevant to your niche. They may have an area where you can drop off promo materials like flyers, business cards, etc. You'll also be able to organically start conversations there with people who are into your topic, which'll make it easier to mention your show and hand over a business card. Make sure your card includes your website URL and mentions all of the popular places it can be found (Apple Podcasts, Stitcher, etc.) so people can look for it where they're already listening to podcasts.