Any HD or Lowes, thou i find their sections to be out of order and never stocked. Normally have to ask them to look it up in the computer and check boxes on top. Or you can order online, depends on how fast you need it.
the one in manhattan probably doesnt carry it b/c no1 would buy it- i think HD wanted that store to be a little more trendy then industrial, thats why everything is made for Apts. who knows
some LFS sell plumbiing supplies, they just dont tell you, they keep it in back for their service jobs, but if u ask, they normally will sell it