"Joe" wrote in message

Also, I want to use nails. Screws are just plain out of the question,
and I dont want the expense of renting or buying staple equipment. But
the shortest ring shank underlayment nails are 1 1/4", which means
every one will be punching a hole and splintering out the bottom of my
subfloor, which is not desirable. I know they make 25 mm (1 inch)
underlayment nails but I cant find them in the US (or Canada, so
far)..

What to do?

I think you're screwed. Neihter McFeelys nor McMaster have them so they will
be about impossible to find. I'd buy or rent a stapler. Or go with screws.

http://www.halexcorp.com/halex_underlayment_3.shtml

Fasten the plywood underlayment panels securely to a structurally sound,
smooth, clean and dry subfloor using;

a.. Staples: galvanized or polymer coated 18 gauge chisel point staples
with minimum 1/4" crown or,

b.. Galvanized ring shank flooring nails: 12 gauge, minimum 3/16" diameter
head or,

c.. Galvanized flat head wood screws.
Fasteners should be long enough to penetrate through the plywood
underlayment and atleast 80-95% of the subfloor, but not protrude through
the bottom. Fasten one plywood underlayment panel at a time, starting from
one side and working to the other. Do not lack the corners first. If using
staples or nails, fasten on every fastener mark (4" on center in the field
and 2" on center along all edges. Fasten edges 3/8" - 1/2" from the edge.
Fasteners should be set flush so they do not protrude above the plywood
underlayment.