There are several options.
First of all the very nice Zvezda AoT kits. They offer small sets (3,- € each) & a bigger one (the 85-mm AA kit for approx. 8,- €, at least where I live) in pretty good quality.
6112 - soviet 45-mm AT gun M1937 (53-K) + crew of 2
6115 - soviet 37-mm AA gun M1939 (61-K) + crew of 2
6122 - soviet 122-mm howitzer M1938 (M-30) + crew of 2
6145 - soviet 76-mm infantry support gun M1927 + crew of 2
6253 - soviet 76-mm AT gun M1942 (ZiS-3) + crew of 3
6148 - soviet 85-mm AA gun M1939 (52-K) + crew of 4
http://www.plasticsoldierreview.com/Man ... aspx?id=26Italeri did a very good soviet artillery crew as well.
6097 - soviet 76-mm AT gun M1942 (ZiS-3) + winter crew of 6
http://www.plasticsoldierreview.com/Review.aspx?id=1209This set contains 2 guns + 12 figures altogether, means quite good value for your money.
It is is also available via Revell (set# 02531).
Orion also offer very useful plastic figures depicting WWII "Soviet Artillery Crew" (set# 72041)
http://www.plasticsoldierreview.com/Review.aspx?id=200436 figures in 12 poses, 4 poses for for early war, late war & winter conditions each.
Note that you can also buy these as 3 single resin sets (containing 4 associated poses each) from Haron if you don't need that many figs & prefer the finer resin-casting.
Although none of the mentioned sets is created for your presumably 203-mm howitzer M1931 (B-4), I think the mentioned figures could form a good starting point for your project.
Btw, from which manufacturer is your 203-mm kit?
Do you know that Toxso have announced this B-4 howitzer as a plastic kit for the near future?
Hope this helps?!