standalone 3G modems and chipsets widely available.
'Shields' a bit less so, but they're coming online more and more.
(sellers trying to unload 2G stock while they can?)
Sadly, 3G is typically 80% or higher priced than equivalent 2G product.
(Qualcomm licensing and catching the early market rush?)
In another couple of years 4G shields will start appearing, and should be slightly cheaper than the same 3G item. (no single licence source)
I have a SIM532x shield (w/o GPS) in development right now - at a decent price for my own projects.
It should be available within about 3-4 weeks.
ADDED: Warning! Quad or penta band modems usually only operate on 2/3 bands in each region, and the band/modulation schemes mean that you *must* be careful of the part number variant you select for each country and individual carrier.