Sorry it has taken me so long to look up the reference I was thinking of,
but here is what I have found.
According to "Legacies of the Sword" in a note on the fukuro shinai:
"Kamiizumi Ise-no-kami Hidetsuna is believed to have been the first famous
swordsman to adopt the bamboo practice sword (fukuro-shinai or hikihada)
in the late sixteenth century." (Kamiizumi Hidetsuna [aka Nobutsuna] --
his birth is disputed, "Legacies of the Sword" has it between 1505 and
1510, while "The Samurai Sourcebook" has it around 1520. "Legacies of
the sword puts his death around 1577).
Later Friday writes concerning the practice weapon: "The first documented
useage of this device was in 1563 by Kamiizumi Ise-no-kami Hidetsugu--who
is generally credited with having invented it--and his student Hikida
Bungorou Kagekane in their duels with Yagyuu Muneyoshi. While bamboo
practice weapons enabled swordsmen to train and compete more
realistically, with fewer injuries, they did not catch on immediately...
Miyamoto Musashi, for example, belittled the value of training with shinai
in the opening passages of the fourth chapter of his Gorin no sho." [I
have drawn out the long sounds with 'u' as I do not have the neccessary
character set in my e-mail program]. According to "The Samurai
Sourcebook", Kamiizumi Hidetsugu was the father of Kamiizumi Hidetsuna.
Friday references Inagaki Motou's "Kengou meishoubu hyakuwa", 26-31 for a
detail of the duel between Hidetsugu and Muneyoshi. He goes on to give
reference to an english account in Sugawara Makoto's "Lives of Master
Swordsmen", 96-99 which I cannot find at the moment. (Hmmm... makes me
think I really need to organize my library).
So, the fukuro shinai is period, but was not widely used. For those who
don't know, be aware that the fukuro shinai is NOT a modern shinai. For
one thing, a fukuro shinai is heavier and encased in leather. You should
be able to find them, although I'm not entirely sure where.
"Legacies of the Sword: The Kashima-Shinryu and Samurai Martial
Culture", by Karl F. Friday with Seki Humitake; (c)1997; University of
Hawai'i Press, Honolulu, Hawai'i; ISBN 0-8248-1847-4(alk. paper) -- ISBN
0-8248-1879-2(pbk. : alk. paper)
"The Samurai Sourcebook", by Stephen Turnbull; (c) 1998; Cassell & Co,
London; ISBN 1-85409-523-4