You would call one of your methods in your @Respostiory. The method would return a collection of options to populate your drop down box with. It might look something like this if your drop down were categories.
The database look-up work would be done in myRepository.
Once you have the list your question merely becomes how to populate a selection box.
If you are just selecting a list of Strings it is very simple. If you are using objects there is a little more to it. See this thread from a couple days ago. Pieces of it may be relevant to you.
It sounds like you are doing what the options tag does for you. If you want to do it that way you can but it should be within a select. You also need to use the Spring taglib for your select and option elements if you want it bound to your object.