I defrost my rat/mice in the fridge overnight. Take it out and warm it up using hot water from the faucet. Soak for 15 min. Then throw out the old water and repeat for another 15 min. I do not use plastic bags. I tried that and I find that the feeders don't thaw as fast or get as warm.
Take the feeder out, pat dry with a papertowels and use a hair dryer on both sides of the rat/mouse for at least 30 sec. The bigger the feeder, the longer I stay on the face and sides of the belly.
Your feeder should feel warm, especially the belly area. It should also be "squishy" and not hard. If it feels warmer than your hand, it definitely is at or above 90's F.
Then immediately offer the prey to the snake. Using the hemostat, grab the hip area or the skin above its hindquarters of the feeder. I don't grab the tail because they almost always break off. Same with the legs. One time I accidentally grab the belly of a pinkie and 'gut juice' popped out. Gross.
I have never stick a thermometer probe inside a feeder. I have used it on the feeder to check temps. But now I don't do that anymore as long as I follow the above steps.
If you thaw and blow dry the feeders near or in the same room, your snake should be ready to eat when it is time to offer the prey. You should get a strike within seconds. If it did not, wait 15 min, reheat with a hair dryer, and try again. If it still refuse, then stop for the day and try again next week.
I buy my feeders from Underground Reptiles. Their store is only about 15-20 min from home and they are cheaper than the other pet stores near me, including the online vendors. But I only have 3 snakes to feed. If you have a large collection and a good size freezer just for feeders, I hear good things from Perfect Prey. However, I do agree Petsnart overprice their feeders so I would never buy from them.