How do I convince my channel partners I won't steal their leads?

You can’t. A channel partner who refuses to share customer information with you has probably been poached by another vendor at some point in the past. Instead of trying to convince them you won’t steal their leads, focus your energy on building trust with them. It won’t happen quickly. Trust seldom does. But as your relationship develops, your partners will determine whether or not they can trust you. If you can then show them value they can enjoy by sharing leads, prospects, or customer information with you, they’ll likely do so willingly.