Most of the Chambers of Commerce will have coupons at their information booths. I'm not sure where you are staying but the boats out of Provincetown almost always have coupons at the information booth in Eastham which is just past the Orleans rotary on Route 6 East (The road is not divided so if you miss it you can turn around and go back).
Another way you might get coupons is to write to the tourist information boards and chambers of commerce in the Cape Cod area. Many of them will send out packets of information that include coupons if you sign up. Here's the link for Eastham Chambers of Commerce.
http://easthamchamber.com/visitors/get-guidebook
I just signed up to get the book myself as we're going there in a few weeks