Well I don't think US snipers dressed very differently from standard infantry, so I think your problem is simply getting a correctly scaled sniper rifle which you can then just use with any US grunt in the right posture.
I believe the Tamiya US WWII 1/35 weapon set has sniper rifles.
If you want Brit/Commonwealth I think you will find that SOME of their snipers wore denison smocks and maybe scrimmed their helmets a bit more, but still fundamentally wore standard uniforms, so maybe get your self a paratrooper figure for the denison smock and put a standard helmet on him Unless of course you want a paratropper sniper!).
But even if not, I think a normal infantry man in battledress with a hessian covered scrimmed helmet would do for a commonwealth sniper.
You can do a reasonable scrim with putty, you just sort of put it on thickish and string it out.
Not sure where you'd get a No4T (sniper version of no4 rifle) but I don't think they would be too hard to scratch build.
I am also pretty sure that the US weapon set mentioned above has at least one or two loose scopes so you could take one of those and adapt it to a NO$ if you weren't 100% fussy about having the correct Mk32 scope on your No4T.