You should receive an email immediately after your purchase with your ticket and receipt. However, don't worry if you can't find it, all of your entry details are stored on your Racer Dashboard in your Race Space account.
To access your Racer Dashboard, make sure you're logged into your Race Space account, then select 'My Entries' from the main menu. You will see all tickets purchased under this account on the My Entries tab.
To find your ticket, simply click 'Entry Actions' next to the ticket you'd like to access, then 'Download Ticket' from the Entry Actions menu.