There's a bunch of options on the scope rail, the H&R is the most popular, I'd have to put the Leupold in 2nd place IMO, at $5-$6, it's a real bargain too, but doesn't come with a hammer extension, but for another $7-$8, the Uncle Mike's can be had, it's what H&R sells, Midway sells the pair for a reasonable $14, but their surcharge adds $3 if that's all you order.
Tim