There are two different possible scenarios:
1) The spring & plunger securing the seatback is missing but the seatback is in otherwise fine condition. Click here to see the “symptoms” of a missing spring and plunger. If this is the case you can order a replacement at that link or at your local specialty retailer.
2) The seatback is somehow broken which is very rare but can happen. If the seatback is cracked or the pin on the opposite side of the plunger is broken then the entire seatback may need to be replaced. It is only available through specialty kayak retailers. Reference part # 18500572. If you feel that only a few parts must be replaced, please send a photo to our customer service department and we can try to provide some alternatives.