Locally, salvage yards and classified sites such as Craigslist are excellent starting points for finding VW motors. Online, hundreds of viable options ranging from eBay, Volkswagen-specific retailers and many more are available.
The primary advantage of purchasing locally via classifieds or a salvage yard is avoiding shipping fees, which can be quite substantial when dealing with an item as heavy as a car motor. Many also consider it prudent to search locally before resorting to an online purchase for such a large item. However, when purchasing through an online retailer, enthusiasts are more likely to find exactly what they are looking for.