Is it possible to download iOS 8 beta without Developer Account? My device is iPhone 5 (American version). Is it possible to get iOS 8 beta without buying access to Developer Account?
No.

You need to be a registered developer agreeing to the license agreements AND your device needs to be registered as a development device.

As always, there are ways around this, but that is illegal. Torrent sites sometimes offer the software and in the comments developers offer to register your device as a development device. But these developers are at risk to lose their own development access.

Furthermore the first betas tend to be buggy. Unless you have an obsessive need for the latest stuff or are a developer you don't actually want it on your phone.