You can get them from Hotwire Savings Club (
http://www.hotwiresavingsclub.com/hsc/index.asp ) The ones on eBay are usually ok, but the problem is that the lunch ones cost $17 or more, and they only have a value of about $15. You might be able to get $17 out of them, but if that is the case, then just pay cash for your meals. I have used them, they work great.
The certificates are from Disney, but they are sold in volume to conventions and the like. You can't buy them as an individual from Disney, but other places sell them. The do say Disney right on them.