Tin is where you find it. Anymore I've resorted to "biting the bullet" and buying it straight from online purveyors. In the whole scheme of things the cost isn't prohibitive in terms of cost per bullet, and I know exactly what I'm getting. That helps in creating consistently repeatable alloys from batch to batch. Lo those many decades I spent concocting alloys from wheelweights, range scrap, printer's type, and unknown crap, usually with just a thumb nail test for hardness, that sometimes worked ok and sometimes not - but never with repeatable results. In recent years I've codified my alloys, striving to stick with simple tin/lead stuff whenever I can and life is simpler.
As for yard sale and antique shop pewter it's a crap shoot as to what you're actually getting. No guarantee of the precise percentage of tin there, putting you squarely in the "by guess and by gosh" realm of alloying. Another consideration is the value of the stuff - always check the hallmark on it (which it should have if honest pewter) to make sure you don't melt down something that is worth hundreds (or thousands) of dollars in order to make a hat full of bullets. Note also that ancient pewter had a really high lead content too - imagine that, drinking out of a lead vessel, and then wondering why your hand doesn't stop trembling.
