Well, you got a number of options.
If you like cable machines, you'll like this. Use the pulley up at the top of the machine and the tricep rope attachment. Parallel to the machine a few steps away, stand with your feet just a little wider than shoulder width. Grab the rope with both hands and concentrating on the obliques, rotate your torso and bend down, the motion being very similar to chopping a tree with an ax. The key is don't use a lot of weight. Seriously, doing three sets of this with proper form is a killer, so don't use a heavy load. I use only use about 15 or 20 pounds, and that's effective.
You could also use dumbells to work your obliques, but again, not a lot of weight, but additionally the range of motion is key. You may see others do this exercise by letting the weight bring them down to one side at the start and then bending to the other side at the end. That's a no no. Contract your obliques and bring yourself up so that you are standing straight at the ending position.
That same exercise you can do on a lower back extension bench, provided it has a ledge where you can put your feet to provide enough balance. Same thing, contract your obliques and bring yourself up so that your spine is straight; don't hyperextend. Hyperextension can cause added stress on your spine.