They don’t have one yet, but when I signed up to test the CF it was about two and a half weeks before it showed up, and that was during the Christmas rush. (Caught all of the volunteers completely by surprise when it showed up.)
If you don’t have a filter already on order, you would need to specify to Glowforge that you want to buy one somewhere on the sign up form. They might be able to allow a few new customers to buy any extras that they have from a production run after they have satisfied all of the pre-order requests.
They will bill you separately via PayPal if you’re selected to receive one, but you will still need to fill out the sign up sheet to be considered for one now. If you have a few months breathing space, I’d go ahead and sign up for it, and make sure to list on the form that this is a new purchase, and you need to be billed before a certain date. If it starts to get close to your cutoff date and they haven’t gotten a filter to you in time, you can send support an email at email@example.com before the date and cancel the order.