Please take what follows in a constructive manner.
It seems strange that you are looking for "purest" alloys for seveal reasons.
For one thing, an alloy is a mixture of metals, so it is not "pure" in the sense that a single metal (say lead) is pure. Lyman #2 is composed mainly of lead and tin, but will almost certainly contain some "impurities" of alimony and arsenic. Those additional metals are actually beneficial if you want hard metal, because they allow hardening by "heat treating".
On the other hand, some "impurities" such as zinc are to be avoided because they will keep the alloy from properly filling-out molds, leading to imperfectly cast bullets and loss of accuracy. I doubt that this is a problem with anybody selling "alloys" for casting. But, if somebody is selling "wheel weights" (or ingots that they cast from wheel weights), then zinc might be a problem, because some wheel weights are now made of zinc and those must be separated from the lead ones.
One alloy that some casters like a lot is "linotype". It is a mixture of lead, tin and antimony in very specific proportions that was developed for casting printers' type, because it fills molds extremely well and allows for clearly detailed characters on the faces of the type used to assemble plates for printing newspapaers. That means that it will also cast very pretty and accurate bullets. But, although linotype is hard, it is also more brittle than other lead alloys that have been heat treated to make them as hard (or even harder). So, for hunting bullets, linotype may not be as desireable because it can shatter on impact and not penetrate as well.
There are also some "proprietary" alloys that include other metals such as silver. The Oregon Trail Bullet Company touts its "silver bullets" that contain about 2% silver.
"Pure" (or almost pure) lead is very soft. That is sometimes a problem for bullets used in cartridges. But, it is a good quality for bullets that will be used in muzzle-loaders. So, some people are looking for "soft" lead to make their balls or bullets for use in flintlock and caplock muzzle loaders.
Most people who cast bullets are trying to shoot more for less money spent. They most often use scrounged lead, including wheel weights, discarded printers' type, lead liners from shower stalls, etc., etc. They may add some tin to increase casting qualities, or add some alimony to increase hardness (usually using lead/tin solder or even a little shotgun shot). But, the idea is to make a lot of decent quality bullets for very little money.
However, sources of lead to scrounge have become increasingly rare, and several vendors are now selling alloys to shooters. By the time that you pay for alloy and the shipping required to get it to you, you may be better off just buying the bullets cast and sold by somebody else, especially if you are not going to shoot a very large number of them.
So, until you can tell us more about what you intend to do with the bullets that you plan to cast, it is hard to give you advice about what would be "best" for you to use or do.
SL1