You could try etsy, ebay, your own website, and other suggestions. It's not a great time to be making new vape equipment without a business license, honestly. Given the direction the FDA is going and etc. You could be liable if a minor purchases your creations, for example. So, I am hesitant to recommend options, as really, if this is a side business that isn't really being run "as a business" it does open you up to increased liability, although right now I think the stakes would be lower, but one never knows when that will change.
Anna