Note, a retina scan is not required for Nexus. You can apply for Nexus in any border city - it includes GE and gives you pre-check. You can use Nexus at any land crossing without a retina scan. You only need the retina scan if you want to FLY into a Canadian airport and clear using Nexus, and you can only get a retina scan at a Canadian airport (or I believe there are downtown offices in Vancouver and Buffalo).