Populating listboxes for each record in datasheet view

I'm pretty new to access and entirely self-taught. I've hit a typical 'Know what you want access to do but have no idea how to make it do it' problem.

I have a form from which users can select the Order No of a particular order.
Each Order No relates to a number of Order Items.
Each Order Item relates to a number of payments against it.
In a subform is displayed each of the payments made against each of the order items in datasheet view.

If for example I happened to be using the database to run a fruit shop then each Order No would represent my deal with an orchard to buy fruit from them, order items would be apples and oranges and payments would relate to buying the apples or oranges for March, May, June etc.

For each record in the datasheet I want users to be able to select from a dropdown that displays the order items that relate to the particular order No in the main form.... and have no idea how to do it.

Any help would be much appreciated.
Mark (who is considering the fruit selling business because database design can be such hard work)

This being a nice slow and warm summer day, if you wish some help, post or email me a zipped sample of your database with a couple of samples and I'll go at it. The problem is simple enough but I believe in "seeing is believing".
ineuw@aei.ca