Karen, I don't know what they can get over the counter, but I do know a lot of vets recommend NOT sedating them at all. If it's a long trip, the cats will eventually calm down and will settle in quickly. As long as the humans are calm and relaxed, the cats should pick up on that....hopefully.

Just let them know if they don't get anything, that it's not the end of the world. 300 miles isn't that far, and they will all arrive a little tired, but ok in the long run.