I don't know of a specific way to search for batteries by specs over multiple vendors. You pretty much have to shop the brands you like, then search the specs on the available packs.
I really like Hyperion G3 packs. ALLeRC is a reliable source and has great customer service:
The specs on that pack are:
3S 2100 25C
L - 105mm
H - 22mm
W - 34mm
Weight - 163 grams
Very close to the stock battery for the Extra 300. Sure they're $30 (which is still fairly inexpensive), but you get great bang for the buck.