Gabe,
I do not have any specific recommendations on filters for your situation, although I think I have seen such things advertised. You may not find them at the large distributors like Digi-Key or Mouser because they tend to be specialty items. The other issue is that there is not a "one size fits all" solution. As Mike pointed out the power line frequency interference may be in the middle of the desired signal bandwidth. The "damage" to the desired signal due to the power line interference filter needs to be evaluated for each case.
Some people use phase locked loop techniques to track the actual power line frequency (which varies slightly) in very narrow bandwidth interference rejection systems.
You indicate that this is a recurring problem. That suggests to me that you are not using best practices in your measurement systems. The time and money spent trying to remove that later might be better spent reducing the amount getting in in the first place.
Lynn