"We at (place your inspection company name here) have arranged for certain third-parties to make special offers to our clients, offers you may want to take advantage of. By signing this agreement you permit these third-parties to call you to discuss these special offers. (place your inspection company name here) may be paid by these third-parties for acquiring this permission. Please let us know if you find them to be of value so that we continue making them available to our future clients."
You can use this clause to create a stand-alone permission form that lists all the third-parties or you can just incorporate the clause into you main home inspection agreement.