eBay 2.5 to 3.5 IDE

[amigakit]

It depends which way you want to go but if you wish to add a 3.5" device to the A600/A1200 2.5" IDE interface then this cable will work:  http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=209

Ignore the photo - you can specify any length upto 25cm at no extra charge depending on what circumstance you wish to use it in (just state length in order comments)

[amigakit]

Oh and don`t forget that depending on which way you are adapting to re-crimp the header onto the cable the correct side or you will have a burnt cable.