First I'd consider *which* Cossacks were drawn on for the advances into Transcaspia and Turkestan in the 1860s through 1880s, and then look at sources for *those* Cossacks. The Taras Bulba image isn't quite right; these were properly uniformed troops with modern weapons for the time.
For example, for Turkestan in 1882, look on pages 312-313 here:
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Then look up the Ural, Orenburg, and Semirechie Cossacks starting on page 96. Then note their *uniforms* (referring back to the Don, not the Circassian style) beginning on page 121. Also see page 118, and someone please find me any models wearing the armiyachina!
Copplestone's BU36 are closest he makes for these regiments, but don't reflect the local adaptations made for conditions in Central Asia. Askari's Don Cossacks are as close as you're going to find for what you want:
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If I'm pedantic, it's from doing a thesis on this subject, and poring over all the available annual editions of this document (and many, many others), tracking the buildup of Russian forces in Central Asia over time--with an eye to gaming it someday, as well.
Allen