Which online “opportunity” sites are recommended? (Upwork, Task Rabbit, LinkedIn, etc.)

In general, I do not recommend using “opportunity” sites to handle your marketing for you. The reason is that it encourages a commodity mindset on the potential clients and supports the belief that all bookkeeping service providers are the same. You want to position yourself as different than “just a bookkeeper” and provide superior quality services. Price shopping clients often undervalue our services and can be difficult to work with. While you may get some experience using these services to build your confidence, it will put you in a position to compete on price, which is a losing race to the bottom. Better to get experience offering your services to charity where you will be more appreciated. Just my opinion.