TommyG... Yes, it will work harden quite quickly. Make sure you anneal frequently. Mike mentioned working with coin silver, and I do the same. Those always have to be annealed first, since the dies work harden them when they are stamped at the mint. Also, fine silver is much easier to work than coin silver, since it is much softer. I never use nickel silver, and all front sites I have ever made are .900 coin silver. This gives them the toughness to resist bending, and show nice and bright when drawing a bead.
Frenchymanny... Mike is right... You are unlikely to find any on the market for silver stars, because there is no market for them. You will have to fabricate your own from sheet silver. Not sure when Canada stopped minting coins in silver, but here in the USA 1964 was our last year for coins in circulation to be minted in .900 silver.
Matt