Search for milwaukee m18gg-502c.Pricing up a Milwaukee grease gun right now,you don't really get the whole kit(gun,battery and charger)you have to buy individually!
This is the grease gun kit which includes two 5Ah batteries and charger. There's a few sellers in eBay too. Expect to pay about £350.
I think it's cheaper if you're after a hammer drill to buy the batteries with that, it'll cost less than the extra £200 for them with the grease gun.