They were shipped with win3x?, what do you define win3x as? if not a referrence to the whole 3.x series, such as we define windows 95/98 as the 9x series.
there is not a windows version 3x, it would be 3.0, 3.1, or 3.11WFW.
What CPU does the Amstrad have? MIPS?
You might want to try ebay, or a local computer show.
You will not be able to legally obtain it via downloading from the net, as it is illegal. So you might want to check into a online store that carrys older versions. ebay will for certain.