I want to write an automatic process that downloads an ios app and decompile it? I couldn't find any source or method to download an ios app without using any Apple Device.
AFAIK there is no way to download apps without a device, since the necessary keys are embedded in the CPU and cannot be extracted using software means (this also prevents offline decryption of apps). However, with a jailbroken device it's possible to simulate user input in the App Store app and download the apps automatically (and then decrypt them). Check this presentation (video) from last Recon Brussels for some ideas.