Not sure about the propriety of mentioning specific vendors' products in the messageboards, but The Only Sheet (www.theonlysheet.com)produces an integrated character sheet based on EXCEL that has a separate sheet for familiars/animal companions/mounts. As a character increases in level the familiar increases in strength/ability. In all, a truly excellent product.
There are specific sheets for this in the Advanced Character Portfolio and they are soon going to be redesigned for the final playtest update.