I was thinking about replacing the lithium PRAM battery in my 5300c, because it's dead.  I also bought a replacement NiMH PRAM battery for my impending 3400c.  I opted not to buy the 5300c battery because it's like $15 and it's honestly just not worth it to me.  The 3400c replacement was $7.  So it got me thinking that it might be useful to create a sort of database on replacement batteries.  I'm not familiar with any beyond these two models/series yet, but here were the best (3400c) and only (5300c) sources I could find.

5300c - VL2330 (6v lithium (2x 3v))

http://www.amazon.com/gp/offer-listing/B0000X5RIM/- $9.50 + $4.99

3400c - 7.2v (6x 1.2v) NiMH [60mAh]

http://www.ebay.com/itm/7-2V-80mAh-CMOS-BIOS-Ni-MH-Battery-Pack-/181676870672?pt=LH_DefaultDomain_0&hash=item2a4cc90c10[80 mAh] - $4.99 + $2.00