custom search of .aspx search on website

I am doing an investigation and I need to search a website's tax records for people who paid their taxes on a specific date. When you go to the site, you search their site by their criteria, but they don't allow anyone to search just by tax paid date. I need to do a custom search within the site, I need to search for receipt date within their .asp web file. i know basic functioning of websites, but I don't know aspx or whatever they're using. Maybe a shell script to search a site or something of a Custom program. I can talk on phone if need be To give more details.