30 years ago in Mashiko, Japan I bought two Haniwa type horses.  One
was about 6" tall made of black clay.  The other horse was about 10"
tall wearing a saddle of terra cotta color clay.  Where can I buy
other Haniwa figures?

This possibility is all I was able to find.

Tokyo National Museum Gift shop probably sells Haniwa reproductions. The
Inquiries (by fax): +81-3-3822-0088(Museum Shop fax)  

This is the museum phone number  TEL. +81-3-3822-1111

http://www.tnm.jp/en/guide/museumShop/index.html

?Be sure to check out the museum shop in the basement; it sells
reproductions from the museum's collections as well as traditional
crafts by contemporary artists.?
http://www.frommers.com/destinations/tokyo/A23453.html