There are a few options. You see the larger one from Seidio above.
Another option that doesn't modify the case is the Fuel Case Case-Mate BlackBerry Bold 9000 Fuel - Rechargeable Battery Pack (Case-Mate BB9000FL-BLK), BlackBerry Bold Cases, Holsters, Skins, and Accessories in Premium Leather - Model 9000
That one looks interesting.
A third option is a portable battery pack recharger like one from JAVOedge. JAVOedge - High Capacity Portable Battery Charger Home Starter Kit for BlackBerry (RIM) 9000
I am using the Seidio battery. I got it this week as my OEM battery was dieing. Very impressed so far. I watched 2 hours over my SlingBox last night. Then left the Bold unplugged overnight. This morning I still had 55% of my battery power. Much better than I was getting on the OEM.
I also have the JAVOedge kit. I spend a lot of time on airplanes; 10-14 hour flights. I use my Bold to play music on these flights. The OEM battery was dieing too quickly. The JAVOedge recharges that battery in just a little over an hour. I can get two full charges of the Bold from one charge of the JAVOedge. It does seem to take about 4-5 hours to charge the JAVOedge.
Hope that helps.