I presume you are going to the Monaco Historique Barry.
If so I cannot comment on Nice but there is a reasonable if Ferrari biased model shop (that also sells other ephemera) on one of the streets in the main part of town to the north of the harbour. Sorry can't do better than that as I don't have a Monte Carlo street map to hand.
There is also a harbour side shop in the Port de Fontvieille whose website is monalisamotorsport.com and e-mail
monalisa@monte-carlo.mc that deals in more expensive stuff (helmets, bits of old F1 bodywork, books etc.)
Meanwhile hope you enjoy the racing, and by the way Saturday's weather forecast is suggesting some wet stuff, I wonder how Stuck will cope with the Auto Union.