Welcome to the forum Danaile.
You have a long trip ahead of you, but lots of people on this forum have made the trip and I am sure will chime in. First, do you know which puppy you will be getting? The reason I ask is that if you do, the breeder can start taking you puppy out in the car for a few minutes at a time to let him/her get used to the car. Each day she can add a minute or two so that at the end of three weeks the puppy will be really used to the car.
Here are some if the things you will want to bring with you.
Paper Towels and regular towels (in case the puppy get sick)
Nature's Mircle stain remover to clean up the mess
Water bottle and a bowl (to drink once you stop for a break)
Food and food bowl (just in case, you never know)
I also recommend a cat seat, you can strap the puppy in, they can see everything and be close to you. Here's a link to the one a lot of us have:
As far stopping at the rest area, I wouldn't worry about pee pads, but just walk around the area where there are no dogs and do his/her business.