Greetings,
In my experience, these are typically very hard or impossible to come by from the manufacturer. Its not considered an accessory, so they are not normally stocked independently of a board. eBay is a possibility, but again, you'll probably have someone wanting to sell their board, not an I/O shield.
The supplier is responsible. Some deals are not "so good" that they shouldn't be passed up. If it is, then you'll have to accept the missing part or return it. MB's are not something I ever buy used. The amount of money you might have saved, is not worth the time, energy or what you are going through now to save a few bucks.
I hope you find one, but it's not going to be easy.