Good AT BX/ZX based motherboards like the ones mentioned above are nearly impossible to find online. A few S370 AT mobos I have come across are these:
Soyo SY-7SBB
specs buy
Asus ME-99B
specs buy
a cheaper one:
PC Chips M726MRT
specs buy
this one has both a socket and a slot on the same board
You might be better off just buying a cheap ATX case like
this one for $23, and a low cost i810 board such as
this Tekram one, for about the same price as one of those AT boards.