Rather depends what you want. There exists an old Dinky, prewar 3" AA gun that is a reasonable fit for 28mm. Surprisingly they aren't necessarily that expensive if you are prepared to search long enough.
If you fancy making your own, the the Minifigs 4.7" gun makes an ideal barrel. The mororised 18/13 pdr (rebored 18pdr)and straight 13pdrs were extensively used by Britain during the war, probably the most widely used AA guns of the war. Sources for the gun(s) itself are quite plentiful, Great War, Foundry, Woodbine, OG etc. Sloppy Jalopy make a Thorneycroft J that is ideal as the mount. you'll find useable plans and photos here:
http://www.landships.freeservers.com/13pdr_AA_on_thornycraft_walkaround.htmIf you want something lighter, you could always pair the Eureka Hotchkiss gun, complete with shield with a Fiat ter, i know someone makes one, maybe SJ. These saw service around London during thw war, I think one or two even made it to Belgium.
Th easiest thing to do for Germans, French and Russians is to get the appropriate model field gun and make a lashup like the ones used during the war, jerry built turntable upon which the gun, field carriage, wheels and all was mounted.