If you’d like a normal LED matrix display with a (generally) dark backing, those are pretty easy to find. On the other hand, if you’d like an 8×8 display with just LEDs and electrical connectors, this kit from Jollifactory is a rare find that will suit your needs quite nicely.
As designed, the display will measure about 144mm x 144mm (or almost 6″ x 6″ if that’s easier for you to think about) square; instructions and build videos can be seen here on. You do need to solder the martrix yourself and LEDs aren’t included unless you select that as an option, but you can use standard 3mm, 5mm, 8mm and 10mm LEDs as long as they have leads of at least 23 mm long.
The display is driven by a MAX7219 IC, which allows several of these units to be daisy-chained together to form one large display, using just three digital microcontroller outputs. You can see the results of this setup in the video below.
On the other hand, if you’re thinking that this kind of display could make an excellent component for an LED cube, they’ve thought of that too. Be sure to check out the jolliCube, or Tindie’s blog writeup on it here for a more “cubed” version.
