The first place to look for your tickets would be in your email inbox. All receipts are automatically sent there. You can try searching "BetterWorld" or "[email protected]".
The good news is, even if you can't find your email confirmation/receipt, the organization will have your purchase and tickets on their Attendees List!
The second place you can find a receipt is in your BetterWorld account. Here's how:
Head to My Account, and click on the product that you're looking for a receipt for (i.e. a donation receipt, a ticketing receipt, and so on).
Click View next to the transaction to see the receipt.