At the risk of sounding glib, it depends. Not all lhbs are the same.
Whole grain is usually sold by the ounce or pound.
Dry malt, usually sold in prepackaged amounts... 1 pound, 3 pound, 5 pounds and larger. Not all lhbs carry all the different prepackaged weights.
Liquid malt is usually sold in a prepackaged can. 3.3 pound (1 kilo) being the norm. A larger lhbs may carry bulk LME that you can purchase in different weights.
And if you buy kits, you will already have the malt in the amounts needed for that recipe.
Some lhbs will sell you just what you want, others may not have that option, and you'll have to choose from what they have. The only way to know for sure, is to visit one, or several, and ask.