Been looking at OG & Redoubt. I'm sure that there are others
James - some of the Perry's American Revolution stuff may be usable, as may some of the Foundry range... I've generally thought Redoubt may be the next best - although their figures are a little large sometimes (similar to Front Rank); I have some of the ACW stuff, and used to have some Confederation of the Rhine Nappies (originally painted by Roly Hermans of Arteis/Dressing The Lines fame)... They are/were all notably bigger than Perrys, Foundry, etc... Redoubt quality however is a bit better than OG IMO - there's more consistency and none of those dreadful square hands or heads... Dixon also do a F&IW range - and many people rate them highly - but personally every time I've bought Dixon stuff about a third of the figures have looked awful... And size wise they are like Redoubt & Front Rank, with some suffering from balloon heads...
Front Rank chaps are very large and only really mix with Redoubt.
@McBeth has mentioned Conquest above (I haven't seen them in person but believe they are in the Perry/Foundry/OG size range); and there is also Galloping Major range for some unique/personality types to add variety - some of the figs are outstanding others less so - but a few packs mixed in with another brand could give a really nice variety...