Steppe Cossacks (Don, Astrakhan, Siberian, Amur, Transbaikal, Ussuri, Yenisey, Semirechensk) all wore identical uniforms to the regular Russian infantry of the period. The only noticeable differences at 28mm scale will be the colour of their shoulderboards, breeches, caps and some wearing a Cossack style papakha. Most of these can be ignored as well, especially during WW1 where even the Cossacks tended to not wear coloured items frequently; though they did make a rapid comeback into fashion during the RCW. Even the Caucasian hosts (Kuban, Terek,) along with their Christian Caucasian allies largely wore standard uniforms in the field.
However, I have an inkling feeling you are looking for the classic Cherkesska clad Cossacks. I remember quite well a site that does them, though for the Russo-Turkish War of 1877. However, the uniform is almost identical to that worn during WW1/RCW so the only necessary conversion will be for the rifle. I'll see if I can dig up the link somewhere.
EDIT: Found it,
Outpost Wargame Services. Just check the 'Ranges' section and the War of 1877 gallery should be there.
If you are looking for dismounted Steppe Cossacks, than I recommend
Copplestone Castings BoB range and
Brigade miniatures Storm in the East. Both have excellent WW1/RCW infantry that can easily be used as Cossacks