The cellular carriers available to a given device are determined by two things: a) radio type and b) device type. Particle's SKUs represent both of the information in one number.
Finding Available Carriers By SKU
Simply put - go here: (link).
This link enumerates available cellular carriers after selecting a given SKU by radio button. You can see that by selecting any of SKUs/groups of SKUs triggers a list that specifies country followed by carriers. For 2G/3G SKUs, you can note that approximate 2G/3G Sunset Dates for a given country are listed as well.
The first radio button "Electron 2G (G350), Electron 3G (U260/U270), or E Series 2G/3G (E310)" has an extra component to it - "Model." Please note that The Electron U260 and U270 have different radios and will only work in the countries for which they are specified.