Seems like you have to log in to your Roxio account in order to access the 7.1.3 update. I bought the Toast 7 disc at least 5 years ago so I didn't remember ever creating an account but I tried various old e-mail addresses at the "forgot my password" page until one was eventually recognized. Then, they just e-mail you a link to reset your password.
I started here and then clicked "Download Toast 7 Update". Then, if you're already logged in, it should take you directly to the download page. If you're not logged in, it probably first asks you to log in and then takes you to the download page. I ended up with about an 85MB zip file.