Various recommendations for figures for Early Serbian or Croatian on the DBMM List. The initial question was about figures for the 6th - 8th century.
In no particular order:
Dave Turner:
The Prussian/Lithuanian foot from the old Gladiator Feudal range now available through Black Hat Miniatures would have a lot to recommend them. The range includes an Warrior with Axe figure essential for Croatian Armies.
They may not be specifically Croatian but I doubt that matters too much.
This range also includes the worlds best 15mm Peasants so if you want to build some of those "Ill Armed Foot" they can also help out.
Duncan Head:
By the 8th-9th centuries, at least, they look pretty Carolingian.
Peter Feinler:
Outpost and Irregular both have a few Slav figures.
There's also a couple of Slav figures in the Tabletop Byzantine range - see this link:http://www.15mm.co.uk/The_Byzantines.htm
Lorenzo Mele:
I've bought some figures from Essex dark ages range, and Essex, Donnington and Old Glory slav ranges.