It is hard to find a non OEM replacement for the under the rear seat mounted batteries. The under the rear seat mounted batteries require a special vent. Last time I checked, only GM carried them (dealer item).
Checked Interstate'* web-site, they do carry a battery for under the rear seat (Bonneville), it is :30 Month
840 CCA (Cold Cranking Amps)
Part # MTP-79
Interstate lists $160 for the battery, same cost as the GM dealer. I am not sure how readily available it would be at Interstate. For me, I would go with the OEM from a GM dealer.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.