The same as my suggestions for Pages - www.iworkcommunity.com or, believe it or not, Microsoft's Office site.

http://office.microsoft.com/en-us/templates/FX100595491033.aspx?pid=CL100632981033, has links to many PC templates. Then comes the Microsoft trickery part. Choose the category then sub-category & then click the name of a template. Next is a page with a "download" button that takes you to the "Your operating system doesn't meet system requirements" page. All that means is that you can't use their automatic download service. Click the "download now" button in the section to manually download. Somewhere along the process was a license agreement. The file I chose downloaded as 06369101.cab when testing Word downloads. Double-clicking it launched Stuffit & resulted in a folder. You will have to change the Excel template extension to .xls for Numbers to be able to open it. You may get a warning or two, but the template should function.

The Mactopia site, http://www.microsoft.com/mac/templates.mspx, has a lot less templates to choose from, but was less involved to download. Most templates for Excel will open nicely in Numbers. I haven't come across one that doesn't, but that doesn't mean someone else might. The only thing to watch for is the file extension. Some Excel templates have an extension other than .xls. You will still be able to use them, just change the extension to .xls in the Finder or Numbers won't recognized them as Excel files. Of course, you can't change just any file's extension to .xls & have Numbers open it. It must be an Excel document.