How is gelatine sold in U.S. grocery stores?
The packs typically contain the equivalent of 15mL of gelatine and are in powder form.  Each pack is measured to set 500mL or two cups of liquid.  As far as weight, it feels like about 6-7 grams of gelatin.

The dominant brand seems to be knox.  



As for the bloom, I'm not really sure as I haven't seen it published on the label.  However, I have actually made marshmallow using the same Alton Brown recipe and it seemed pretty tolerant.  So slightly different bloom should be fine.  I also deviated with the corn syrup and didn't use it (I avoid corn).  The end result was still awesome.