In the end I went for the following combination:
Helmet purchased with Belsavis commendations
Chest purchased with Corellia commendations
Skirt purchased on Belsavis from speciality goods vendor
(approx 40k credits)
All orange gear, all then needed to be modded up to suit a level 50 ranged DPS companion (though if ever I decide on a better look for Andy, I can happily make Talos my new Imperial Guard with no immediate need to change the mods). All three carry the words "Ardent Blade" in the name, though they are of Venerable and Primeval "quality" each. I guess this prefix denotes the level of the gear, so I suppose there's different versions of each floating around.
As has been pointed out, these are by no means an exact match for the Imperial Guard NPCs that you occasionally meet. But I figure that a) most people won't notice the difference, b) this gear actually looks a bit better anyway since it has gold trim, and c) it probably makes more sense that the uniforms for my own personal guard would be somewhat different.
Anyway, it looks pretty fantastic
. Now if only I could persuade Kaliyo to squeeze into a suit of standard Imperial Trooper armour...