hi there and welcome
heres a page that may help you with the size of hole to cut and also if you wanted to have a go at making your own
Spiral Stairs or if you wanted to buy one then try here
DOLL HOUSE MINIATURE-SPIRAL STAIRCASE KIT-METAL-NEW on eBay (end time 01-Mar-10 08:46:28 GMT)
SCK1 - Spiral Staircase Kit
at the bottom of the page you will see spiral staircases here too
Magpies Miniatures