have you tried the insurance company where your other business is? If they already have your house insurance, then they may give you a quote.
the area where the site is may have a bearing on the quotation, ie if it is near a built up area and chilren tend to play there or could be attracted to play there, then the risk is higher. Also, bear in mind that when you commence construction, you will need different insurance covers. As far as I know, if you intend building in the next few months, one of the self build policies may be of interest to you.