What, SPECIFICALLY, is "wrong" with your liver?
Are certain enzyme levels elevated?
I do not know if you realize this or not, but heavy weight training can elevate levels of certain liver enzymes. This is just a normal effect of training hard.
The only way I could imagine a protein POWDER causing your liver trouble would be if the protein powder you are using contains something else in it which your liver cannot effectively deal with. Again, this is if your liver is showing true markers of damage.
You mentioned no other supplements, but what about meds such as acetaminophen and others?
Another possibility would be due to some sort of inherent defect you have which is unrelated to protein powder but related to inability to metabolize certain compounds ( eg, copper in hemachromatosis.)
If protein itself were causing the problem, simply eliminating the powder isn't going to help. All that beef and chicken and fish I assume you eat would cause the problem too.
So again, to reiterate, there are far too many variables here to simply say "protein powder is bad for your liver." It would be wise to get off of the powder for now, however, in your process of eliminating possible causes.
In regards to your antioxidant question, this is easy: fruits and veggies. Vitamin C, Vitamin E, all sorts of polyphenols in fruits- eat a multitiude color of fruits and veggies. You can indeed buy antioxidant pills, and there is a plethora of them out there. Do a little searching and I'm sure you can find one that is to your liking.
But still, I'd be interested in hearing the specfics, if you know, of whats going on with your liver.