De-cat Pipe - 02 Sensor Help

[Ed_Tsport]

Hi, my brother has made me a de-cat pipe out of a spare cat and midsection he bought off eBay, as far as im aware fitting this will make the eml come on. i read in a previous post that a 02 sensor step out device is needed so the light only comes on when there is a real problem, but where do i buy one of these from? a google search gives no car related searches :D