We have an already very complex form that represents a contract between a dealer and multiple retailers. Until now, choosing the Retailers was done using a multiselect dropdown with a search textbox to filter results.
The problem is that the list of retailers has grown by A LOT and now there are way too many options to populate the dropdown list. The list takes way to long to build, both on the server and the client side.
I thought of a combobox that only populates it's list after the user types something in the box, firing an AJAX call to the server to retrieve retailers based on the entered terms (just like when you enter tags for your questions here in stack exchange). This would solve all problems but one: the user will have to make one search for each retailer he wants to select and, depending on the quantity, it can become a nightmare.
I don't know if helps but it's an ASP.NET MVC5 web project and we have Telerik's UI Tools for MVC at our disposal.