ATX Adapter

[amyren]

I think the only ATX adaptor kit made for sale is that adaptor to use ATX powersupply on A1200.
http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=771
Not in stock though.

The rest I think will be a DIY project. Here are some examples, but perhaps not made with ATX cases.
http://www.amibay.com/showthread.php?9670-Amiga-A1200-Tower-Project
http://free-zg.t-com.hr/dozanic/
http://www.cosam.org/projects/a1200t/

As for commercial options, this is the only A1200 tower I have found for sale:
Elbox BlackBOX 1200.
http://eu-shop.elbox.com/en_GB/p/BlackBOX-1200/734
It is expensive though, at 300 EUR. No wonder why people want to do it on their own. And to make it useful, you probably should add a PCI adaptor board as well. And then you would be ready to buy the next useful stuff like a gfxcard, soundcard, usb-card, ethernet-card. Now you are close to 850 EUR. And you would probably also need an adaptor for connecting an exernal keyboard as well.