If they have additional filters from an order, they might very well allow people to purchase them. I didn’t have a filter on order, but I offered to buy one so it could be tested.
They will bill you separately via PayPal, but if you’re wanting one, go ahead and sign up for it on the form, but explain the circumstances (you don’t have a GF filter on order but want to buy one now) in the writeup.
It requires a lot of additional manual fixing on their end, so they might not want to do it yet, but it doesn’t hurt to put a request in, and it gets you in line. Eventually they were planning to offer these as an alternative to the GF filters. 