Walmart and Amazon are both in a great position to do this! Setup an online store that allows established customers to suggest items that they are unable to find on local store shelves. Allow them to also flag other submitted items as interesting. You have a curation staff that fleshes out submissions with images and descriptions before they go live. The company's merchandiser evaluates the balance between interest in an item and the difficulty of its procurement. To aid in appropriate pricing, perhaps the system also requires submissions to include a reference to the most similar item that is already stocked on store shelves. Because the company is offering exclusive access to the items the profit margins can be kept high.
From what I've experienced product selection in US stores is in general far better than even the same store in Canada (including Walmart), but there might still be a few items not stocked in local stores that people will pay a premium for!