I really like Perfect Prey.com - their f/t rats are always clean and don't show signs of over population like barbering (over grooming from stress that causes hair loss or skin sores) or having urine / feces on their feet and tails, and no signs ff URI like crusty noses or eyes.
They are a little more pricey than some of the online feeder providers but I've been very happy with the quality - also their "size breakdown" is a little more varied than sm/ med/ large which gives more options in my opinion
Back when I first bought from them I emailed to find out how the rats were processed and they use the carbon dioxide method, which leaves no chemical residue for your snake and is one of the faster ways to dispatch mass rodents with as little stress as possible.
I'm sure for the rats its not a completely stress free method, but its better than the way most of our own food (chickens / pigs /etc) are dealt with.