Limiting Vendor Selection

Hi all,

I have created a custom search form which displays inventory items by vendor. I've included some filter fields like 'Inactive', 'Type', 'Special Order Item', and 'Preferred Vendor'.

We have about 250 vendors, but only 20 of them are current stock suppliers, which is what this list is for. So when you click the drop-down 'Preferred Vendor' list at the bottom of the list, I'd like it to only display those 20, not all 250 of them.

I've created a custom Entity field on vendor records called 'Current Stock Supplier', and this is ticked for just those 20 records. However I'm stuck as to how to apply this to the search form. I've specified a criteria as 'Current Stock Supplier = true', but while that limits the items shown, it has no effect on the drop-down list of vendors.