Most transmission parts suppliers will sell parts to individuals. This is the route I would go. If you don't want to go that route, there are some online places that you can get what you want from as well.
Transmission parts & Transmission kits
http://www.bulkpart.com/
Northern Auto Parts: - Transmission Overhaul Kits
Transmission parts & Transmission kits
http://www.bulkpart.com/
Northern Auto Parts: - Transmission Overhaul Kits