I also purchased one of these screens from displaysscreen.com. It is the correct size. It does not come with a driver board though - you'll need to source one of those. The link posted by jopamo (above) is a good US domestic source (i.e., no long wait time for shipping from Hong Kong) for a compatible driver board. Search ebay for "180979602491" and you'll find the source I bought mine from. Very good vendor, and if you communicate with them after ordering and tell them which panel you have, they will make sure that you get a driver board that will work with it.
I haven't yet built my Picade because I've been learning about this topic and have been assembling parts. Once I get everything done, I hope to create a comprehensive post on this topic. Until then, allow me to contribute some of what I've learned.
First, there are other threads in the Pimeroni forums on this topic. Here are two of them:
An important thing to note about upgrading to a 12.1" screen is that it requires a different video driver board than that which is included in the Picade kit with 8" display. It also requires a separate and additional 12VDC power supply, unless you are handy enough with electronics to add a power converter module so both the Pi and the 12.1" display can run off the same power supply. You'll also need to determine the amperage needs of the 12.1" display and driver board, and make sure that the power supply you use will supply enough power to it.
The screen you will want needs to be a 12.1" 1024x768 screen (4:3 display ratio). There are various model numbers you can look for, including N121X5, LTN121XT, HT121X01, LTD121ECNN, and LTN121XL01. The easiest way to buy a 12.1 LCD screen and compatible controller is to order them together. Search ebay for "170947783291" for an example.
Note, however, that there are two different types of backlighting used on these panels: CCFL and LED. LED is better than CCFL in certain ways (more modern tech, uses less power, longer backlight life), but I haven't found anyone who ships a LED-backlit screen (12.1" 1024x768) and driver board combo, and buying one of these LED-backlit screens plus a separate driver board will be more expensive than buying a combo with a CCFL-backlit panel. CCFL-backlit panels require an inverter board along with the driver board - as long as you order the right board for the right panel, it will be included. Personally, I purchased a LTN121XL01 (LED-backlit panel) and a separate driver board because (1) it's the combination that Pimeroni originally offered and (2) I wanted an LED-backlit board. CCFL screens can be significantly cheaper, however - check the model numbers on amazon.com - you can often get screens for $50 or less.
Finally, you can cut the shim that comes with the 8" panel to fit a 12.1" panel. Or, you can contact Pimeroni, and they will send you a 12.1" shim free (shout out to the excellent folks at PImeroni for their SUPERB after-sale support!).
Setting up the Picade with the supplied 8" screen and driver board appears to be super simple. Replacing the 8" panel with a 12.1" panel is a bit more complex, but it's an excellent learning experience. I'm already planning another project to use the leftover 8" screen and driver board - cigar box Pi laptop anyone?
Good luck with your upgrade!