I would suggest you practice right-hand patterns.
Your note that the exercises you found are "useless" suggests to me that the area you truly want to improve is your right-hand finger picking technique.
There are 3 fundamental patterns you should master with the right hand to begin with:
1) Ascending arpeggio:
Thumb, Index, Middle, Ring.
2) Descending arpeggio:
Ring, Middle, Index, Thumb
3) Ascending and Descending (played as a sextuplet pattern; 6 notes in a beat).
Ring, Middle, Index, Thumb, Index, Middle
Now, the best way I know of to practice this pattern is to play a major 7 chord like this:
E A G D b e
x x 12 11 10 9
(You can play this pattern with all three arpeggio types above).
Now, you can get a left-hand work out (and make the exercise more interesting) by stretching one note lower every time, like this:
E A G D b e
x x 12 11 10 8
E A G D b e
x x 12 11 9 8
And continuing until you've stretched all the way back to first position.
Let me know if this helps. There are many other exercises that could be useful but I think it's a good idea to start here.
This advice also applies to a pick-style of playing, but in that case the fundamental methods used to play pick-style guitar fall into three categories.
Alternate picking is a method of playing that emphasizes a strict rotational picking of down, up, down, up. Chromatic exercises on the guitar with four notes per string are a good way to practice this. The most effective application of alternate picking will be on a single string. the "Flight of the Bumblebee" is one of the best repertoire pieces to work on alternate picking.
Sweep picking is playing several consecutive notes using the same picking direction. By necessity these notes must be on adjacent strings. One of the best ways to practice sweep picking is by playing major and minor triad arpeggios across all strings.
Economy picking doesn't get as much attention these days, but basically it's alternate picking on single strings, but using sweep picking to get across strings. It's very economical, hence the name. One of the best ways to practice economy picking (besides difficult repertoire) is to play three-note scale patterns across the strings, taking care that each time you switch strings you use the same picking motion on the next string as you did on the previous. It's sort of a mini-sweep.