In 28mm there's the range of
Strategia Nova, an Italian manufacturer. I got some sets of them
here.
Quality-wise they're a mixed bag with the General Staff set being pretty good.
Also Warlord Games makes
a set of Paracadutisti with German weapons, which - with regular army helmets - would fit perfectly, as they're wearing the RSI-style trousers.
Generally pre-armistice Italians would work, but you really want to modernize the trousers into the baggy RSI-ones with green stuff: Should be easy enough, you just need to get rid of the puttees. Also on some miniatures you could replace the headgear with a German field cap, as the RSI adopted a very similar design.