Lets just say if the fish gets splashed about, which it will as its impossible not to on a journey in a car, then a bag is much softer than the sides of a box, it will be fine, just be careful and avoid as much sloshing about as possible.
Lucy there is every chance that along with the internal infections/worms (or whatever it appears the fish has) and the stress of the journey that the fish may die unfortunately, hopefully not but its always a risk transporting sick fish. Which is why Wilder has been asking if its not possible that the vet would just look at a photograph rather than put the fish thru the journey.
Any small corner shop will probably have sandwich bags or similar, they are really easy to find.