If you're talking about something like the big fucking G&P PEQ2 then anything. No one uses that anymore though lol, I have one on my SOPMOD but I've since switched to a PEQ15.
If you're talking about the smaller PEQ15 then hobbyking is your friend. Although I've heard they have problems shipping Lipos through regular mail or something but I'm not sure if that's been dealt with. I have the specs written down somewhere but it's not in their "airsoft battery" section. I'll see if I can find it though.
It might have been this one but I can't guarantee its the one that fits.
Someone else mentioned this one is known to fit:
In the same vein this should also work as well:
Note that you'll have to solder a new connector onto the end of the battery. I recommend Deans plugs but small tamiya will work as well (I think the VFC comes with small tamiya).