There are several maple vendors at Union Square, and
they all carry maple butter. The one I go to (which
also carries fiddleheads in season) is called
Deep Mountain; they're based in Vermont (arguably the
finest maple syrup with the strictest quality
standards) and they offer the maple butter in a few
different sizes, as well as several flavors of maple
candy: blueberry and ginger, to name two.
I know that Deep Mountain is definitely there on
Saturdays, and may be there Fridays as well.
Maple butter is an intermediate product between the
very liquid syrup and solid maple sugar, sort of a
thick creamy maple spread.
The maple products vendor(s) at the Union Sq. Farmer's
market (and sometimes Grand Army Plaza and other
markets) may sell it, but Im not sure. The vendor does
have multi grades of syrup and kinds of maple sugar
lumps (ginger, walnut etc)
There was also a store in Carroll Gardens at Henry and
Sacket which sold maple and other vermont products,
including maple butter, but I am not sure its there