you can still go ahead withe the showpopup - you just have to change the popup properties:
eg if your calendar controls are on frmCalendar and your textbox is txtCandar then this will show the popup form directly below your textbox:
frmCaendar CalendarPopUp = new frmCalendar();
CalendarPopUp.Width = txtCalendar.Width; //optional setting that one
Currently what is your Calendar Controller - a UserControl?
if so you can easily convert it to a form by editing this line of code at the top of the CS file:
partial class UserControl1 : UserControl
partial class UserControl1 : Form