Oz,
There are a couple of approaches you can consider here.
I am not sure what exactly you mean by "sub class of the stock number", and
how this sub-classing is recorded in your database. But it is a reasonable
well-used ideea in database design to have two comboboxes, one of which you
select the subclass, and then the second only shows the items related to the
category selected in the first. This is sometimes known as cascading
comboboxes. This is probably a lot neater than trying to manage it on a
separate form.
The other thing you should be aware of is that by setting the Auto Expand
property of a combobox to Yes, you can move to a section of the listed items
based on typing the first characters of the desired entry, and this often
allows comboboxes to be used effectively, even when they are based on a big
list.