You do not have to look for a carrier overseas that has sim cards in the new micro size. There are several firms that now sell 'micro sim cutters' for maybe $20-30, which will nicely chop your standard mobile device mini sim down to micro sim size. Or you can do it yourself with some scissors and a paper template for a micro sim; there are a number of YouTube videos showing you how. Then just pop it in your 3G iPad and you should be good to go with respect to cellular data service.
In case you are wondering about the very idea of swapping in a sim card from a carrier in another country, Walter Mossberg, the Wall Street Journal's tech reporter, was recently asked a question from a reader about using their iPad overseas -- and Mossberg then queried Apple about this. Here's the exchange:
Q:
If you buy a U.S. 3G iPad and take it to Europe, can you swap in a 3G SIM card from an authorized iPad carrier there to avoid AT&T roaming fees?
A:
I haven’t tried it, but Apple says the answer to this is yes. The company told me that if you insert a SIM from one its partner carriers, the carrier settings get set automatically when you plug in the SIM, though in some cases you might need to have connected your iPad to iTunes recently for this to work correctly. Apple adds that you can even use a SIM from a non-partner carrier, though in that case, you’d have to manually enter the carrier settings on the iPad. Note, however, that the iPad uses a new, smaller size of SIM card that not all carriers sell yet.
Once you solve this last sim card size-problem, as I noted you can easily do, then there should be no difficulties.