Part number U8735.  You might be able to purchase them through Dell, but you can get them from many suppliers: [URL] [URL] [URL] 
Hi,  We have a Dell PowerEdge 2950 that is reporting a failed PERC 6/i battery. Where can we purchase a replacement battery? This server is no longer under warranty. Thanks, Shang-Lin
PowerEdge 2950 Perc 6/i battery failed