Hi Lindie,
One way you could do this if you populate the dropdown value from a secondary data source (2DS).
See the sample template I created, if you preview the template and put in A, B, or C you will see the dropdown values change.
The steps to do this are:
1. Create your secondary data source - I did this in Access and exported as XML
2. Import into your form template as in data connections
3. Add your text field that filters the drop down
4. Add the dropdown, bind it to the secondary data source
5. Add a filter to the dropdown when you bind it to the 2DS so that the values shown in the drop down are FilterName = text box value
One limitation to this is that if a user puts in "a" instead of "A" it won't work but you can put data validation on the text box field and prevent users from entering "a", "b" or "c" and force them to use caps or vice versa
Hope that helps
Ted