This flat-type PSU is hard to find it. Because Infinite S is a customized system. but maybe you can try to RTX 4070 or 4060. 4070 only needs 200W. And the graphic card size should be fine.

https://www.amazon.com/Mini-%E8%A7%A3%E6%B1%BA%E6%96%B9%E6%A1%88-Flex-%E5%85%A8%E7%AF%84%E5%9C%8D%E4%B8%BB%E5%8B%95%E5%BC%8F-%E9%9B%BB%E6%BA%90%E4%BE%9B%E6%87%89%E5%99%A8/dp/B08BF1H5R1?ref_=ast_sto_dp&th=1&language=en_US