Try a local lumberyard, not a big box. Or look for a "newel post" (the end or turning post of a stairway banister. Or take 1-bys and face glue them together to make a laminated post.

Hardwood dealers often have large sections of various softwoods also.

Look in the yellow pages under 'hardwoods' and start calling.

Be prepared for the price.
High grade larger pieces of any wood are rather expensive.