How to make a pop up combo box like calendar pop up

I have a form with a series of text boxes for entering order information. But
because i want to be able to select more than one entry from a combo box, I
thought it would be nice to have each item added to a text box and for
neatness and ease of programmability, I'd like it if a combo box would pop up
beside the text box much like the date picker pops up when entering a date
field. Then each item selected could get appended to what's already in the
text box.

So my questions a

Does such a thing already exist and i can just download it and add it to my
database?

or

Is there sample code of how the date picker works so that I can examine it
and attempt to adapt it to my DB

Thank you!