I've been eyeing the Buy Buy Baby GC to turn into Disney. You can use Buy Buy Baby at Bed Bath and Beyond and I've bought Disney GC at Bed Bath and Beyond with GC's a couple months ago. It's YMMV as the system allows it and there's no corporate policy against it, but you might run into a cashier or store that says you can't do it. A small set of us on the Gift Card thread tried it over the summer and mostly no issues but a few had some problems that took a bit to resolve. Also some reported that their stores don't have the 3rd party gift card rack, so I guess you'd want to know if you have a store that even carries other gift cards first.
I'm looking at the Buy Buy Baby because they give 3% but Bed Bath and Beyond are only 2.5%
Others that people mentioned that have worked at some point are Kmart/Sears and Safeway. Again, it's all up to individual stores. Some have reported at Safeway can't do it at self checkout but no problem with a cashier, some stores won't do either, and then other stores no problems anywhere. So YMMV no matter what. I don't have either in my area so no first hand experience here.