Why not buy a kit that provides the bolts?
http://www.ebay.co.uk/itm/5MM-HUB-CENTRIC-WHEEL-SPACERS-SHIMS-BOLTS-FOR-BMW-Z3-Z4-M-COUPE-ROADSTER-SB-/230945684177?pt=UK_CarsParts_Vehicles_CarParts_SM&fits=Model%3AZ4&hash=item35c56f7ad1
Just a thought...... there are others