You can order a "world sim" online from Brightroam.com. The cost of the card itself is nominal (less than $5.00) and it will function for 90 days. You can extend that to up to a year for an extra cost. You are charged for use strictly on a post paid basis, i.e. they charge your credit card for actual calls and texts (no data), the charges varying with the particular zone you're in.
I've used one of these and although it did end up costing me substantially less than if I'd been roaming with my home (Canadian) SIM card, it can still add up to quite a bill. So it's fine for occasional use but I'd advise buying a local SIM card where possible if you actually plan to communicate extensively, especially if you're likely to make calls as opposed to texting.
Brightroam also sells some country specific SIM cards which generally work the same way.
I used a full size SIM but assume the smaller ones are available.
I'm sure there are other vendors as well.