There is a few places online you can sign up for to purchase casino credits. One is instadebit which will give you instant credits in your casino account. They do limit the amount you can purchase but these limits will be raised the longer you use their webwallet solution.
The way it works, is it directly debits your bank account the following bank day the amounts you purchase. So you must ensure you do have enough to cover the purchase or you will have bank charges and return charges at instadebit. All you have to do is download any online casino ( as most have instadebit as a payment solution ) and within the banking section of the casino lobby ( sometimes known as cashier ) you can sign up in there.