The gun you linked to with the regular plastic haggard should be able to take most batteries. You could use Nimh ( nickel Metal Hydride ) batteries or newer Lipo / LiFe packs.
The Nimh are bigger and lower performing but easier and safer, in theory. A Nimh Mini Battery should fit in your choice of gun.
The newer tech batteries like Lipo's are supposedly a bit more dangerous and have been known to catch fire if abused. Don't let this frighten you as long as you use a suitable charger and watch how you charge it,preferably on a hard surface or in a Lipo Charging bag which you can buy for a fiver you will be fine. Also do not subject the batteries to abuse, such a puncture or bend them. Lip's perform a lot better than Nimh so do not go above a 7.4v pack, it will perform the same or better than a Nimh 10.8v. An 11.1v Lipo will make you gun run fast with great trigger response but it will also kill you gun a lot faster!
One other thing to bare in mind, never fully discharge a lipo or you will kill it, a lip alarm will prevent this, it simply plug into the charging lead and an red led/audible beep tells you when to stop using it and discharge.
This is the battery I use, you may find a larger one will fit but I know this will fit easy ( infact you could carry a second on in the gun ready to just swap the connectors over.
http://www.component...po-battery.html
The charger I use iirc or a very similar one
http://www.component...e-with-psu.html
The charging bag ( not essential but good for piece of mind ).
http://www.component...-bag-large.html
Lipo Alarm, this one does not have audible beep, so just check it when in safe zone or at lunch.
http://www.component...-7-4-14-8v.html
Any decent airsoft shop should be able to advise you and supply similar gear and if bought along with a gun there is a good chance of a discount. If not Component shop a good to deal with.