Reaper's Chronoscope line has some corporate troopers that stylistically remind me a lot of Imperial Marines, especially in the helmets:
Pig Iron System Troops would work in the body, but the helmets aren't right:
If you could find 28mm medieval sallet-wearing separate heads, you could easily convert these, as the heads do not come attached.
For that matter, you could even get some WOTC Star Wars prepaint stormtroopers and replace the helmets and maybe weapons. In a pinch they would work.
I have been long looking for a 28mm option myself!