Is there a particular reason why there isn't a Still Spell rod?
I get that you need a free hand to use the rod which might be seen to negate it's purpose, except for the fact that Stilled spells ignore arcane spell failure from armour and shields.
Another potential benefit a Still Spell rod could provide is increasing the DC of the Spellcraft check to identify the spell.
Nevertheless, a rod would lose enough of the benefits of the actual Still Spell feat that they didn't bother making one. You're free to house-rule one in, though; I'd give it a very low gp cost.