Lyman's #2 is a good mix, but it it sort of wasteful because it uses more tin than you need, and tin is the most expensive component of your casting alloy. The tin doesn't harden your alloy - it is added to allow the alloy to fill the mold completely, which only requires about 2% tin (as opposed to Lyman's 5% content). I used to use the Lyman formula, but upon the advice of others here at TFL I changed my formula. What I began to do was to mix 1lb. of Lyman with 1lb. of WW. It essentially cut my use of tin in half, but there was still a sufficient amount of tin to give me good mold fill out. As far as other tin sources go, you can use lead free solder (95%tin, 5%antimony); I use solid, not acid or flux core. I usually find it at hardware stores for about $15/lb, but the next time I buy tin it will be from www.buffaloarms.com
--they sell 1lb. pure tin bars for $7/lb. When I run out of my already-mixed Lyman alloy, I will begin to add straight tin to my WW until I get just enough to still provide mold fill out. I sat down with my calculator to figure out the amount of tin to add to a given amount of WW, but I don't recall all the numbers right of the top of my head and I don't have my reloading log with me, so if you wish, e-mail me to remind me and I will give you those figures.
In the mean time, you could mix your unknown alloy with increasing amounts of WW until you reach a level where your bullets don't completely fill the mold. Keep track of your WW-to-unknown ratios so that you can reproduce your results. Labgrade made a perfect point. If you used 2lb bars of 50/50, you could mix 1lb of WW to 1lb of your alloy to approximate the Lyman formula.
There are a number of threads here on this issue so try a search for those and you will find a wealth of information.
Hope this helps, and good luck!