Hi,

I would recommend you to modify bizform web part as it is described in Developer’s guide http://devnet.kentico.com/docs/devguide/index.html?modifying_web_part_behavior.htm
You could add combobox into bizform and depending on current selected item change the Form name of BizForm.

Another way could be based on adding query parameter into URL when selected item in combobox is changed instead of customizing BizForm WebPart. And then display appropriate BizForm depending on current value in URL.

Best regards,
Michal Legen