I've found Silver Half Dollars, Quarters, Dimes and War Nickels in circulation here in the US.
It should be possible to find Silver ½, ¼ and 1/10 Balboas in change in Panama.
I imagine you could do the same with Canadian Quarters and Dimes in Canada and the old Silver centered 10 and 20 pesos in Mexico.
I found a 1920 1 Franc in a lot of modern Swiss Francs sold to me near the exchange rate, so I assume it's possible to find Silver Francs in Switzerland.
Australia's decimal coins are still the same sizes as the silver coins they replaced, so it might be possible to find one.
Before the introduction of the Euro, the Netherlands (1 Gulden), France (5 Francs), Germany (5 Marks) and Austria (5 and 10 Schilling) still had base metal coins that were the same size as Silver coins of the same denomination, so it may have been possible to find Silver in circulation there, but I have no clue what the on the ground reality was.
Edit: Doesn't Britain make Silver 50p coins that are the same size as the circulating versions? I suppose one of those could theoretically make it into circulation if someone were careless enough to break one out of its commemorative packaging and spend it at face value.