nonprofits like Taproot Foundation (I have first-hand experience with them) and Catchafire serve as intermediaries between volunteers and orgs across the country looking for help. small orgs aren't likely to have funds to bring in consultants, so there's a huge need for this kind of support!

virtual volunteering is an option to explore if there are no local orgs. rather than hands-on support, the focus is on donating professional skills (writing, marketing, biz, tech, etc) to help develop org infrastructure/resources/processes.