That's a great offer, and I'm sure there would be businesses who would like your help. Have you established yourself as a consultant? I mean the mechanical part of setting yourself up, ie, website, business license, linkedin profile, etc.
Remember that just because you are offering your service at no cost, you still need to be able to tell the story of why someone would want to work with you and let you "inside." For example, are you just starting out? are you looking for references or testimonials? be sure to use your personal story in your message.
LinkedIN would be a good place to start. But first, try to get super narrow in your targeting of the type of organization you want to work with. The narrower you can make your target, the easier it will be to reach them.
Find the type of org you want to work with, then you can use sales navigator to reach out to those businesses.
Hope this helps!