Dogg backpacks can be found at the larger chain petstores, or the ocasional smaller store.
I don't know what the US equivlent is, but i got a doggy backpack for only Â£2 at a car-boot sale. (My mum says they're a bit like flearmarkets crossed with garage sales)
As for stuff to bring, travel water and some toys, you may need a blanket depending on if you share with them and what the weather is like where your going. You can forgo bringing dog food if you're up for campfire cooking them some meat/fish and rice, but i guess dry food bags are probably easier.