I bought some at Home Depot--not every store carries it but the ones in the older town (with old clay brick houses) did.
Otherwise, find a local masonry restoration company--the kind that repoints and fixes historic brick. You may be able to buy some off their truck for the cost of "refreshments" or they can point you to the right place.
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
"Preach the Gospel always, and if necessary, use words." --Francis of Assisi.
"Family farms work when the whole family works the farm." -- Adam Klaus