The only way to limit the number of resources shown is through the Assign Resources Dialog and a filter on Group.

You could, for example define your resources as:
Copper-A
Copper-B
Copper-C

Then enter Copper in the group field. In the Assign Resources Dialog, click the [+] next to Resource List Options and select the Group... filter. Then enter "copper" and it will only show you the resources with copper in the group field.

The other option is to start typing in the resource name field in the split to scroll to those resources.