The simple way is to order cash online from travelex and pick up at T5. There is one in the middle of T5 airside. If you are going tot eh BA lounge then order from the kiosk next to the BA lounges (never a queue).
In NCE like others have said, take the tram or a cab. both take cards. I mostly paid by card in restaurants without a problem. Most of Nice is walkable or tram so you don't really need a lot of (or any) cash.