These guys have the original batteries on sale for $259, but I think you have to pay shipping.
https://invertersrus.com/product/lifeline-gpl24t/
Also, to answer OP's original question, I just had mine replaced. I took it to Windish RV in Fountain, Colorado, which is an authorized Airstream service center. They charged me $250 to replace the batteries. I thought this was excessive given it took them less than an hour to replace them. I will likely not go back there in the future.