You could try Upwork. There's Fiverr, but they're pretty terrible, with a business model that incentives canceled contracts. As a buyer, I've been using Upwork for almost a year now to work with a few awesome developers to build an application for my business. You could also run ads through Google and Facebook, which make it easy to target people who might be interested in your services. Putting together a website to help automate some of the tasks involved, like getting contracts signed, filling out forms, etc, could also be a big benefit, especially if you can rank on Google. For SEO, you just have a blog section where you or a paid writer release weekly articles about inventing, making prototypes, monetizing your ideas, etc. You could build such a website yourself with Wordpress for less than $100, or hire someone on Upwork to build it for a couple hundred.