If someone out their makes signs, a magnetic bumber sign would be nice. Sold to volunteers for a resonable cost.
I just volunteered to gather signatures and came across some of the ad/campaign material. You folks should put QR or barcode images on bumper-stickers, fliers, everything! Most smartphones have the ability to scan such digital images with an app installed. That could immediately point to your site or wherever/whatever and conveniently save the specific info to the phone for later retrieval.