Do you mean stores which permit you to purchase a Disney GC with a store GC? Or are you talking about exchanging unwanted GCs for Disney GCs? Totally different transactions. In a purchase, you're getting an even dollar exchange. In an exchange, you will lose some value from the GCs that you're turning in.
I'll assume that you mean purchasing. Offhand, I'd say Walmart (including Sam's Club), Best Buy, Bed Bath & Beyond and Target (in store, not online). And in each case, YMMV. There may be others and you should check the Disney Gift Card Deals thread for more info.