Im am assuming you are using an ORM of some sort so what you could do for your data that is bound/loads into the drop down is use linq.
So you'd have something like
var dataFromDb = new List<yourentity>();
var data = dataFromDb.Take(100);
In order to load them 100 at a time, to get the next 100 you'd need to keep track of what the current count it is on and then do something along the following lines
var currentCount = 2;
var dataSetSize = 100;
var dataFromDb = new List<yourentity>();
var data = dataFromDb.Skip(dataSetSize * currentCount).Take(dataSetSize);
How you go about keeping track of currentCount is up to you but this is the general idea to achieve what you are asking.