You will find them in the entrance to most supermarkets and also in the Parks. When you get your park map, they are shown on them (note that the first one in MGM is actually outside the gate otherwise you have to go right over to the back of the park).

We use our switch cards to get cash and have used our credit cards with no problems at all. I think your bank charges you but it's not much - maybe someone else can fill you in here. I use them for convenience and don't take much notice of the charge [:I]