You should try to make skeleton. Luckly, that one can be dome from anything. I have good results with tin foil and paper-mache, I guess first would help You too.
To be honest in 99% cases putty/clay don't quite matter. There are techniques that would force me to buy specific clay but... it's 1%.
I go with cheapest epoxy most times (Milliput or Pattex).
Actually I think putty You seek can be Milliput, "Standard" is made from Yellow+Grey.
There are several types of Milliput but to be honest You want Standard 99% time and Superfine on 0.5% (0.5% also go to GS since it's IMHO expensive and not that good but with interesting feat - harden like hard rubber - that allow "press mold" sculpting (check Scibor SM stuff, ornaments are "pressed" that way . He have tutorial on his site too.)