Generally speaking you won't find them identified that way in most stores (restaurant supply places excepted). But if you check where they keep the baking pans etc. you'll find racks that almost fit.
F'rinstance, as with one of the links Pete provided for 1/4 pan racks, the measurements are 8 x 10, which is slightly smaller than the pan. A fairly common rack designed for cookie sheets fits almost perfectly in a half-sheet, leaving about an inch space on each side long side, and a hilf inch on the ends.
I did find, in Ace, one that was a perfect fit in a 1/4 sheet. Unfortunately, the bars only go in one direction, and some small stuff won't work with it.