You can find the IDs of your store categories through the Manage My Store page. From this page click the the Store Categories link which will display the top level store categories. At the bottom right of this list click All Categories to show all the categories along with any sub-categories. The category number will be displayed with each category.
US sellers can directly Access All categories by Logging in and going to http://cgi6.ebay.com/ws/eBayISAPI.dll?StoreAllCategories
UK sellers can directly Access All categories by Logging in and going to http://cgi6.ebay.co.uk/ws/eBayISAPI.dll?StoreAllCategories