Evtech,
Just 1 pointer, before asking a question, search search search. Otherwise some of the very talented and highly knowledgable members (not me Im still in the search search search stage) will get a little annoyed as they have already answered your question.
I had seen some threads on these so I did a search for "Power Steering" in the title, this is what I found. Also try searches for Subaru rack or miata rack etc.
http://forums.hybridz.org/showthread.php?t=112003&highlight=subaru+rack
http://forums.hybridz.org/showthread.php?t=89796&highlight=miata+rack
http://www.viczcar.com/forum/index.php/topic,1087.0.html
Also, in Australia the Miata is an MX5, not sure what they are called in the UK though, hope that helps.