In most countries string beans and green beans are exactly the same thing (see wikipedia's entry for green bean). They are both words used to refer to various unripe cultivars of the common bean Phaseolus vulgaris. The phrase string bean is older and dates back to when beans had a fibrous string down the pod that you could peal off. The first stringless bean was bred by Calvin Keeney in 1894, and hence the newer name for this product was the "green bean" to emphasize the lack of the fibrous string. Both names are still in wide use today, even for cultivars where the string is not present (most cultivars). I am unaware of any credible evidence that suggests that green beans are unsafe when eaten raw. Some toxic compounds are present in green and string beans at very low levels (the same compounds found at much higher concentrations in uncooked dry beans). However, note that these trace amounts of toxic compounds are considered safe and that most foods have similar trace amounts of toxic compounds.
So when going into a grocery store, if you see the word green bean (at least in the US) you can safely substitute it for any recipe calling for string beans.
Edit: As pointed out by rackandboneman, the species Vigna unguiculata can be eaten raw and is the usual bean used in "Som Tam". However, when translated to English Vigna unguiculata is rarely translated to "string bean". The most common english name for this bean is "yard long bean", "long bean" or "cowpea bean" (see the wiki entry for the bean used in "Som Tam"). Similar to green bean, there is no credible evidence that such pods are poisonous when eaten raw, and "green beans/string beans" do make a good substitute for Vigna unguiculata in dishes where you plan to cut the long bean anyways.