Installing a lower IOS version than the latest one to a device [closed]

Question

I know that it is possible to install lower versions of IOS to jailbroken devices over various tools by creating custom firmware.The problem is we have some test devices in the company and we want to have one device eactly with IOS 4.3. It will not be downgrade since the device itself has IOs 4.2.1 installed.Is there a documented way to active it and the most importantly where can i find older IOS versions?

Answer 1

EDIT: Editing because this no longer works and it gets downvoted by people who didn't read the comments.

Apple wants us to use the latest version, and develop for the latest version, there is no way of downgrading anymore.

OLD ANSWER: You can download any firmware for any iOS device here

iOS Downloads

To install it, you have to go to iTunes and click on update with the alt key pressed, then you can choose the .ipsw file with the firmware you want.

For the "This device isn't elegible..." message try this