You can very easily make it yourself for XaaS.
Make a hidden field for the values returned by your API call. Make two more string fields: filterText and selectValue.
Create an action that takes array/string and string (your ids and filter respectively). If the filter string is empty return the whole array, otherwise filter the results of the array by the string given and return those values.
Now use the predefined elements presentation binding on the selectValue field and set it to the action you just created.
Now that dropdown will show either all results or the results of the filter; the user then selects one value from the dropdown. As your API results are stored permanently in the hidden field there is no overhead in making that call repeatedly.
This also works if you need the user to select multiple values especially if you use the left/right box in XaaS for display.
I made an example for another user on this forum, I will try to find the post.
Edit: I found the post (VM Visibility - vRO / vRA ). It's not your exact scenario obviously but demonstrates the filter approach.
Thanks stevedrummond, Sorry for slow reply its taken a bit to get this working..
I haven't doe this exactly as your example, but without your answer I would still be none the wiser.
I am now using the filter as per your suggestion. But rather than have a Hidden Field on the XaaS with the results to filter... I have the Action hitting the API and pulling it back and then I am filtering it.
I was struggling to get the filter to work, but after importing your package and seeing how you did it, I was away.
Though I still dont see why the drop downs on XaaS are not the same as IaaS, in being searchable.