The best of course would be to train in Japan. There are three dōjōs, one in Nara, one in Higashiōsaka and one in Nagoya, where training is held regularly. Addresses, training hours and contact data can be found at the Japanese site of the Hōzōinryū.
Outside Japan the only place offering Hôzôinryû Takadaha Sôjutsu training in Europe on a regular base is located in Hamburg/Germany (recently a branch opened in Toronto/Canada as well):
If you tell a Japanese person you are practicing sōjutsu he will usually ask you "sō… what?" If you then say "yari" (spear) he will usually respond by saying "I see! Spear …" illustrated with a throwing gesture.
No surprise. Even in Japan almost nobody knows what sōjutsu is or that sōjutsu still exists nowadays.
As in most koryū (old schools) the line of head masters from the current sōke Ichiya Junzō back to Hōzōin Kakuzenbō Inei who founded the school is well documented.
About the life and doings of the founder of the school on the other hand only relatively litte is known for sure. And the little information that you can find often is contradictory or belongs to the realm of legends.
Keikoyari (training spears) are not obtained easily even in Japan. The author saw a kamayari for sale in a budō equipment shop in Tōkyō just once. And there is only one maker/distributor of wooden weapons who sells kamayari and suyari through his webshop. Recently he ships the spears to destinations outside Japan as well, but they are quite expensive.
12 essays on spear/Hōzōinryū written by Kagita Chūbee (1957 - 2011), 20th sōke (headmaster) of the Hōzōinryū.
First published in the Nara town magazine Ubusuna, 2009.