Sort of custom fileds in drop down

[smird]

Dear All, help is needed!

Use J2.5.9 and VM2.0.20

Is there a way to sort values in custom fields drop down by the number of items in stock for each separate child product?
For example, I have product A and 2 child products A1 and A2. At first I have 1 item of A1 and 3 items of A2, then order shall be A2, A1.
If I have 4 items of A1 and 3 items of A2, then order shall be A1, A2.

Maybe somebody may at least advise a function, which is used to construct and order values in drop down?

Any help is highly appreciated!

[smird]

Any suggestions?