I bought this one on ebay for my hardtop:
http://cgi.ebay.com/ebaymotors/BMW-hard-top-STORAGE-CART-RACK-HARDTOP-Z3-Z4-Z8-318IC_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247QQcategoryZ6763QQihZ024QQitemZ370042201005QQrdZ1QQsspagenameZWD1V
Only $99 plus shipping, and the build quality is very good, too. I can't say if the S2000's dedicated hardtop cart would work with the Z4's hardtop. But my friend who has an S2000 just bought an OEM hardtop recently, but he ended up buying this same hardtop cart on ebay and it worked out pretty well for him.