Carrying a large backpack filled with plenty of food and water, plus a good amount of bullets bought by Jimmy for him, these supplies were enough to sustain Roland for some time as he roamed the Dark Tower world. During this time, finding enough drinking water and food shouldn't be a problem for Roland.
In the living room, Jimmy watched as Roland sorted out his backpack again, placing the bullets he had prepared at the bottom. Jimmy also handed him a carry-on bag to store the clothes he would wear on this side.
Roland changed back into his own clothes. In the Dark Tower world, wearing modern clothes would stand out, and he was already used to his own outfit, which happened to be suitable for the return.
To be frank, Roland's leather jacket coupled with the revolver holster made Jimmy envious as well. When he has the time, he'll definitely get one made for himself, even if just to keep at home without wearing it out.
