You might be out of luck…
Other than the aforementioned blessed book, I could find but one spellbook with more than 100 pages: the Mastery of Word and Thought, a specific spellbook that's "the size of four normal spellbooks" that holds over 300 pages of spells and was the possession of a level 20 enchanter. No explanation of how this spellbook came to be so immense is provided.
…But you really should ask the GM
I think Ultimate Magic could be alluding to the possibility of monumental spellbooks like the Mastery by prefacing the section in which it's introduced with this:
Descriptions, protections, preparation rituals, and spell content can be mixed and matched as desired from different [spell]books [presented in this section, including the Mastery]. Higher-level books can easily serve for lower-level casters—just drop the spell levels that aren't applicable and remove lower-level spells as desired. Similarly, if you desire longer books, combine two [of these spellbooks in this section] together or add spells or formulae of your choice to the desired levels. These books also work as spellbooks and formula books for new characters. Just copy the list, hand it to the player, and go.
(Emphasis mine.) However, despite the boldfaced statement, Mastery remains the lone spellbook presented that exceeds 100 pages. (Other spellbooks exceeding 100 pages described in that section are, instead, actually sets of spellbooks.) So no guidelines are provided for, like, repeatedly gluing the back cover of a spellbook to the front cover of another spellbook to make an increasingly bigger individual tome.
But, while the typical spellbook "has 100 pages of parchment," the specific spellbook Arctic Call has not parchment but vellum, and the specific spellbooks the Mysteries of Shadow and the Manual of Binding have, instead, paper. Further, the sample specific spellbooks are bound in materials ranging from wood and leather to sharkskin and human skin. So, apparently, there's nothing particularly special about the average spellbook sold in the local Tomb of Horrors Gift Shop and Resurrection Emporium—one hundred parchment pages is merely the spellbook standard.
Taken together, this leads this GM to believe that nothing should stop a wizard (if he lacks the time or skill to craft it himself) from commissioning a craftsman to make for him a custom spellbook from similar wacky materials—and, y'know, Mr. Bookbinder, while you're making it, could you add, maybe, like, a couple hundred extra pages? Alternatively, this DM would consider the possibility that a wizards' guild may've sensed the demand for jumbo spellbooks and make them available at absurdly steep prices, especially if that demand were voiced by ambitious, overeager adventuring wizards with too much money to spend.
In other words, there seems to be no mechanical reason to forever limit all typical spellbooks to exactly 100 pages, but ask the GM. All worlds are different.