Once you've found the coach, explain what you're doing, list the rhetoric texts you're using, and ask how your ninth-grader can practice debating skills. The coach may invite the student to sit in on the college sessions. At the very least, he should be able to direct you to an age-appropriate debate group nearby. You can also call a parochial school, if you happen to have a good one nearby. Ask for the debate team coach and explain your situation.
Some private schools welcome home schoolers to extracurricular clubs.
More and more of these are popping up. Ask about the qualifications of the coach before you commit. But these groups are often very resourceful, mounting regular competitions and even statewide championships for home schoolers.