To me, your question is a tough one to answer because it's more of concept than it is somewhere that you can point to on the fretboard. You want to know the b7. The b7 of what chord?
Are you currently going through the GTMU course? Not trying to be snarky with the question, but that concept is in there. They key is knowing the notes on the fretboard (I think Griff has this in several of his courses). What you need to do is take a look at the concept in Lesson 3 (the whole step, whole step, half step lesson) and do that exercise. Using the first answer, you should have written
C, D, E, F, G, A, B, C.
Then go back and write the numbers 1-7 over the top of those, like this.
1, 2, 3, 4, 5, 6, 7, 8/1
C, D, E, F, G, A, B, C
Then it just turns into a matter of applying that lesson to the chord shapes against the notes on the fret board. You're playing a cowboy chord for C. You're playing a C, E, and G, so you're playing the 1, 3, and 5. You want to add a b7? If you don't know it off the top off your head, look at your chart. 7 is above B, so a b7 is going to be Bb. Now where is Bb in the scale on the fret board? That's where the notes on the fretboard come into play. Now plug that note into your chord.
Then it's just becoming familiar enough with the chord shapes to know where the root is. Once you know the root, you just do the whole step/half step dance (GTMU Lesson 3) and then work it from there. Or, if you're playing that C-chord shape, that b7 note is always going to be in the same spot relative to the root.