Strongarm Bracers, Magic Item Compendium pg. 139. It’s 6,000 gp to wield a weapon as if you were a size larger without penalties.
In most cases, that’s an average of +1 damage, which not amazing, but if you’re stacking size bonuses it can be worth more than that. Certainly, in all ways, better than the terrible Monkey Grip feat. You should never take Monkey Grip, since a feat is worth quite a bit more than 6,000 gp (and way more than +1 damage), and because with Monkey Grip you still take penalties, it just lessens them.
As for whether or not they stack, strictly speaking they would not. Both say that they allow you to wield a weapon one size category larger “than you are,” not “than you otherwise could.” Since neither actually changes your size, if you are say Medium, Strongarm Bracers allows you to wield Large weapons with no penalty, or Monkey Grip allows you to wield Large weapons with a −2 penalty, but they don’t interact or stack.
Seeing as Strongarm Bracers explicitly do not stack with Powerful Build (racial trait of Half-giants and Goliaths that allow them to behave in some ways, including for the wielding of weapons, as if they were a size category larger), it seems likely that the authors of Magic Item Compendium intended that the Strongarm Bracers could not be used to wield weapons more than one size category larger than yourself.