You can buy chatr voucher card at any convenience/drugs store or Wal Mart/Best Buy.
Make sure you keep the receipt (paper) with the 12 digits PIN to enter by pressing *233* XXX XXX XXX XXX, ("#" key) and the "SEND" key
Buy the way, the best time is to pay on your due date, because if you add the money before, so it will sit in you "balance" : that is mean you have extra money and if you use any extra pay per use charges before your due date, so the system will take the money in your balance an you will be in trouble.
Let me explain: if you have the current 25$ with 100 international sms/month and if you send more than 100, it will charge you 25 cents/each from your balance after.
So if you have 25$ in balance (because you pay in advance) so it will become 24.75$ (one extra sms) and when the due date come, your line stay "expired".
Then since chatr is a pre-paid company, so there is some solutions for that to cover the 25 cents missing, is to add 5$ by credit/VISA debit card by calling *611 or *3455 from you phone (if your credit card information is already in your file) or the last option is to buy a 10$ chatr voucher card and if you add 10$ so your balance will become 9.75$ remaining in you account and you can use your phone to call or send sms and the 9.75$ can be use for extra services or you can keep at least 5$ in you balance for the next month, so you will need to buy 2x 10$ card to cover your next month.
Have a good day!