A Excel's custom number formats are stored in the workbook in which they were made. The easiest way to make your custom formats available in all new workbooks is to create a template with the formats and then base all your workbooks on that template.

In a new workbook, define all of your favourite custom number formats. Then select File-Save As, and choose Template (*.xlt) from the Save as type drop-down list in the Save As dialogue box. Name this workbook book.xlt, and save it in the Xlstart directory (usually found in C:\Program Files\Microsoft Office\Office). After you restart Excel, all new workbooks you create will be based on the book.xlt file and will include your custom formats.

Creating such a template is also useful for many other purposes. For example, the template can store your preferred default font and size, print settings, named styles, and other settings such as gridline display.

If you don't want to bother with templates, simply find a cell in an existing workbook that contains the custom format you want, copy it, and then paste it to another workbook. Excel will automatically store the cell's custom number format in your new workbook.

Even better, if you copy an entire sheet (even a blank one) to your new workbook, all of the custom number formats from the first workbook transfer to the new one.