I'm not sure I understand your requirements properly....
do you mean something like autocomplete options where you would type in/scan some substring and the field would provide you with list of options that match the input to select from?
something like this?
https://community.nintex.com/message/52309-re-autocomplete-in-a-repeating-section?commentID=52309#comment-52309
text control
Hi again Marian!
Yes, that basically what I'm looking for, just one single line textbox pulling results in from a list, not another form field. I'm a bit confused as to what variables need to be named where in the example.
Lets say my single line textbox is named "searchField".
In the control CSS class, I've named it "AutocompleteControlClass".
In the custom javascript area, I have all the same as your example except the sources bit is changes to my list named Cranes.
Like how do I specify what column to search?
Nothing is happening when I type in it.
What am I newb-ing up now?
my code snippet there is pure adoption for controls within repeating section. it's not a working solution on its own.
read through the whole thread and you will learn that overall solution is based on SPServices library, and there is as well a link to a autocomplete solution for "simple" text control, including example how to read data from other list.