As far i as i know you can't get them with mutators..? but if your really stuck you can copy one from another map.
They can be made in the editor though - so this is probably the easiest option

- im sure you know most of the brush stuff below, but ill run through the whole process anyway:
1. Click on the builder brush button to create a brush. (size doesnt matter u can resize it later)
2. Select the red builder brush, then right click on the 'add volume' button, and select Karmaboostvolume out of the menu.
3. Move the builder brush out of the way and you should have the karmaboost volume - these volumes are tricky because you cannot rotate the direction of the 'boost' by hand, but you need to use numbers. Oh and now use the vertex editor button to resize the volume to what u like. If you look on most maps with boost volumes, they make it big because all the vehicle must be in the volume for it to work.
4. Click on the Karmaboostvolume's properties, then go to Movement > Rotation. The small red arrow on one of te corners of the brush denotes the 'boost' direction, and to change it you must type numbers into the 'Yaw' field under Movement > Roatation. The values to use are:
0 = Right
16384 = Down
32768 = Left
49152 = Up
As for tilting the boost direction vertically, maby try smaller variants of the above number in the pitch field...but i found the boosts work fine for small jumps when the arrow is flat.
Hope this helps! if it doesnt try looking for some tutorials, maby UnrealWiki

gl with the mapping shinobi