I don't understand the question.
Do you have one dropDown which u need to fill out from a table/entity using a filter, and you want to do this in code rather than using an LLBLGenProDataSource control?
Or
Do you have 2 collection controls, say 2 drop down lists, and you want to filter the second one based on the selection of the first one?
And when you say:
I want to keep this a common function its not right to have datasources floating about.
Apparently this is a web site, are you using the same dropdown list several times at the same webpage, or is it used in multiple pages and you want to retrieve it once?