Gerbils are not good as they will not get to a size that is good for an adult BP. They are bigger than mice, but considerabally smaller than a rat, and feeding a snake 4 times a week or 4 at one time to be equivilent to a good size rat is going to be more expensive than ordering rats.

Additionally if you wish to order online as long as you have a bank account you can use your debit card, some places won't take it directly but will take paypal which can be linked to a debit card, or ask someone with a card to order for you and pay them the cash, if your trying to hide the fact you have a snake from your family they will probably work it out when either they see large families of gerbils which slowly dissappear or styrofoam packages with frozen mice get delivered.