I contacted Teespring about that and @tprk77 is right it appears that American Apparel products are only available from what’s in stock. This is what Teespring Support said:
I do apologize, the American Apparel company has been bought out and they are currently reviewing the inventory as to what to keep and they have not sent out any stock since then.
Unfortunately I think that means that we won’t be able to order Triblend’s this campaign. I’ll work with Teespring to see if there’s another product we can replace it with.