In addition to the wonderful forum vendors who will rebuild your bilsteins the following can also be considered. May save y'all some search engine work.
Bilstein in California will rebuild your shocks. Need to fill out a service form available on their site, send them the shocks. You can pay extra for faster turnaround. Looks to run @ 65-75$ per shock for basic rebuild.
Bilstein
I used an outfit called Stewart Development located in High Point, NC. Bernie Stewart rebuilt the shocks. A basic rebuild cost @100$ a shock but Bernie will also do custom valving for you. He quoted me @200$ per shock for custom valving.
I received the shocks exactly 2 weeks from the day I shipped them. Included was a printout showing rebound, velocity etc. Shocks returned were labeled left and right. I had the basic rebuild performed on my front inserts.
Apparently Bernie performs much work for the Nascar community so his turn-around time maybe impacted by the racing calendar. Saw some google threads where Miata owners had some long turn around times on a group buy, but they sent the guy an astounding amount of work (800 shocks?). I have no affiliation with him, in my case two weeks door to door on a basic rebuild and a local to me vendor.
bernie@stewartdevelopment.com
www.stewartdevelopment.com