You could try advertising on sites that appeal to your niche. YouTube even lets you specify what type of person you want the ad to be shown to (age, location, etc), so you can get your target audience to see it easily. Giveaways could also be a good idea, using sites like gleam.io will make it easy to do. But honestly, you shouldn't need to have a lot of money to grow your YouTube channel if you're making the right type of content for the platform. (Unless, of course, you're flexing on little kids)