On Amazon.de I found this battery. Could this be an option as an affordable second battery. The wiring looks to be a slight problem though.
There is a lot of cheaper batterys to buy if you dare to connect it parallel with the original battery, seems common in China, has found a lot of cheap batterys at Aliexpress for example.
Se the picture below how you connect another battery in parallel;
eye when buying generic batteries. ours are of 17 packages in series. the generic ones are usually of 16S which means that having the same amps exhausts before a 16S than a 17S when having less tension the first one at full load.
Just make sure that inside you will find Panasonic, LG, Samsung or any other proper cells. No name cells are not an option.