here is an example of what i use... and like the most. Its an intellect brothers 9.6v "Custom" crane stock battery. this configuration is 3600 mah, but the mah hours on the C cells can go up to 4500 i believe with out changing form factor. Its just a price difference.
The major notable difference between this batt and its closest competitors are the tabs and the leads connecting the cells.
For example the tabs on the intellect battery are much more solidly connected. on the powerizer batteries there is one lead which can easily be accidentally pulled off.
Also on the intellect brand battery the lead that runs the length of the battery is flat. This makes it ideal for inserting into the buffer tube of your gun and getting the full range of stock motion. some other batteries will have a clear hump on the side of the battery which will inhibit the placement of the battery and the motion of the stock.
Many places you order these batteries from will simply make them in the back from loose cells with a shrink wrap machine and some wire and it can show. Intellect Brother batteries when ordered how i like them are made by the manufacture in china and shipped to the us as a completed product properly spec'd out for airsoft.
Horror stories about the local RC hobby shop and low amperage custom batts need not apply here.
sorry i cant help with getting you any of these things right now (IB Batts), gotta work out a better arrangement for imports. We like to get our customers good prices. Need to reestablish with the china guys because second rate us resellers are no good for us customers.
anyway... hope i did just say wayyy to much! lol