My understanding is that they are in essence Sarmatian style lancers, and Scythian style horse archers. Excellent lines abound.
I highly recommend the Sarmatians by Strategia e Tattica
http://www.strategiaetattica.it/ and the "Sarmatising Goths" by Donnington
http://shop.ancient-modern.co.uk/sarmat ... s-91-c.asp if you are doing the later, Roman dominated period. The kit is noticebly more western, and under Roman and German influences.
For the earlier Hellenistic and Pontic period, standard Sarmatians should do. I particularly like these:
http://shop.ancient-modern.co.uk/sac09- ... -528-p.asp and they are an opportunity to mix in some bronze. It is a very nice mini.
At the very origins of the culture, you are talking about basic Scythians, Thracians and Greeks who had met to - for the most part - trade to great mutual benefit.
The Osprey Men-at-Arms #373 "The Sarmatians 600 BC - AD 450" is one of their best, and will give you all you need to put Sarmatians on the board.