SureDone's built in tool should allow you to quickly and easily find the eBay Category ID Number for you listing, we pull straight from the eBay API so it can't be out of date!
However, if you need to reference another source, perhaps for a bulk upload - or to decide which category will be best you can find all the categories and their ID numbers here (this site is neither updated nor maintained by SureDone): http://www.isoldwhat.com/getcats/