The two options I can think of are to either go earlier or later in time.
So one early option would be Saxons or other "dark-age" type stuff. Right now that would mean Wargames Factory or maybe Gripping Beast. The unarmoured guys might look a little more convincing, as I think mail armour would seem a bit out of place in the Empire. (But that's just my opinion -- other people may like that sort of thing.) You'll be limited to just infantry in this case, as the cavalry will really look out of place in the Empire.
Closer in time would be the Perry Miniatures Wars of the Roses figures. Right now they have a set with archers and billmen. Unfortunately (for Empire players), it's very heavy on the archers. Suitable for WotR, but not so great for state troop selections. You may be able to convert some of them with other weapons/arms.
The later option would be for the English Civil War figures, which would mean Warlord Games. You'd be somewhat limited to pike and handgunners, though you could probably convert some other troops. I think they've got cavalry as well.
In the case of cavalry, most other manufacturers' cavalry will look tiny compared to GW. So you may just wish to stick with GW or some alternate fantasy brand (metal.)
In terms of where to buy some of this in the US, I'd check The Warstore. I've had great service from them in the past, and they carry quite a bit. They've got everything mentioned above except Gripping Beast:
http://www.thewarstore.com/