Well yeah it’s like dealing with any tradesman, some will be as reliable as a used car salesman and some will be superstars who’ll make you a billionaire within a decade; and the rest lie somewhere on a spectrum between both extremes. Shop around, speak to 5-10 and choose the one who’s vision and ethos aligns with you and your goals. The good ones aren’t cheap but if they’re bringing in business, then their fee is simply a cost of doing business. How they bring in clients for you is their business. You literally don’t need to worry about it, like using an appliance. If they don’t bring in paying clients, then dump them and try another. It’s such a competitive field that you’re bound to find suitable guys to deal with.

As far as word of mouth referrals go, they’ll be best of course but if you have too much work already this way, then you’ll obviously not need to advertise :)

Regardless, if you’re wanting to grow this business long term regardless of whether it’s part time or full time, I suggest studying up on entrepreneurship, advertising, sales, and related topics. Even free courses online can be helpful here, but your local government almost certainly runs courses for guys like you. Definitely worth looking into.