Lowe's used to have "Roebic crystal drain opener", that's 100%pure lye in 2 lb containers. The store near my house doesn't carry it anymore, but nobody knows if it's a store only decision or if the company phased it out. If you have a Lowe's nearby, check it out.
Otherwise, I bought my lye last time at ACE hardware. The brand is "Rooto-Crystals of household 100% lye drain opener", and comes in a plastic 1 lb container.
However, pound per pound, the ACE brand is more expensive than the Lowe's one.