You can re-send a confirmation email to your customers by getting the initial confirmation from the customer's communications tab and copy pasting it in a new message.
Go to your calendar screen
Click on the booking
Go to the customers tab
Select the customer on the booking and click on their name
Click on customer page
Head to the communications tab
Find the initial booking confirmation and copy-paste it
To the right of their name and contact details, you'll be able to send a message, where you can just copy the confirmation.
If you are looking to send a bulk message to multiple customers on a booking, the steps will be the same where you'll have to select one customer on the booking in order to get the initial booking confirmation, only in this case:
Go back to the calendar screen
Click on the booking again
Go to the customers tab and tick the boxes of the customers who should be getting the confirmation
Click on message