The Storm Trenchers were made into greens, but were never cast that I know of. For my proxies for them, I am converting plastic Imperial Regulars using the pointy Chaos Havoc Jump Packs (blasphemy, yes, I know, but desparate times call for desparate measures).
For the Sterlings and the Black Berets, I would personally use the old Imperial Special Forces packs, and do some arm/weapon swaps.
Hope this helps.
SP