Does anyone know of any font families that 6, 9, 8, 0 look exactly the same upside down as right side up? The 6 upside down would look like 9 and vice-verse. Thanks!
Font's that are very modular, like Variable, are a good place to look. Anything humanist or old style is going to have too much modulation to fit your requirement.
Case in point, try a search for "modular" at MyFonts and Variable and many others that fit your request come right up.
As DA01 mentions, a search for "geometric sans" will give you great results. These are some of the ones listed under this category in Font Shop, the 8s are more irregular, but you can probably find some that are completely symmetric.
ICT Serif Gothic
Futura is a popular one
But the 8 is slightly bigger on the bottom than on the top, now that I look at it in photoshop.