Check these guys out in Moreno Valley, California...
http://cgi.ebay.com/ebaymotors/ws/eB...Name= MERC_VI
Tires mounted and balanced should be an additional $1,500.00 est. (Open Country?) plus shipping.
Goto this website for tire size calculations...
http://www.miata.net/garage/tirecalc.html
good luck,
troy