I'm not as versed in historicals, but if they carry a given era/scale, Old Glory is almost always the cheapest option.
http://oldglory25s.com/index.php?cat_id=158
Further, if you pay 50 bucks a year, you get 40% off everything and they carry ALOT of stuff
Sometimes the main old glory site doesn't have great pictures, but you can often find better pictures here:
http://www.oldgloryminiatures.com
A few other brands to check.
-Magister Militum
-Steve Barber
-Copplestone Castings.
-Irregular Miniatures
A bunch more here: though many are overseas:
http://www.fanaticus.org/DBA/Bazaar/minisources10.html
I've actually had quite good luck ordering minis from the
UK, oftentimes you can get quite reasonable shipping rates, especially if they're willing to drop VAT.
Not to get on your case, but I googled "10mm Ancients" and found all the above on the first page.
:-)
If you're willing to pay a bit more to get better sculpts I'd say stick with 10mm. Some of the figures in 10mm are getting as nice as 15mm figs Sme of that is due to scale creep towards 12mm, but still.
If you want the absolute cheapest, it might be better to go with 6mm, but either way, it's not going to be an expensive proposition.
Best of luck whatever you decide.