Adding tin to any lead alloy won't harden it much. What tin DOES do is increase the fluidity of the melt, allowing it to fill the mold better. It will also make the bullets shiny IF you're casting at exactly the right temp, AND if the antimony content isn't too high.
Depending on where you get linotype metal, it will have a higher percentage of tin in it. It WILL harden your WW, because it's very high in antimony, which is what really makes an alloy harder.
The best source of tin is lead-free solder. Sold in hardware stores in the plumbing supply area. Look for 95% tin 5% antimony content. Some of it will have %5 silver, that's okay, but stay away from the stuff that has 5% copper.
Tin is also available from some other sources, like roto metals, here;
http://www.rotometals.com/product-p/tinhighgradebar.htm
At $12.99/lb it's not cheap. But you only need 2-3% tin in your alloy to get good fill-out of your bullets. Lead-free solder won't be cheap either, you'll pay $16.00 for a 1 lb roll.