Material resources are ones which you are providing. To get quotes and use Comparisons to determine which is the best, you need to be using Quoted Materials.
If you have already resourced your job using Material resources, they can be bulk replaced:
Use CTRL-A or Select All from the Range menu
Under the Range menu, select Replace
Select the Material resource you wish to replace and click Select
Ensure that you are viewing the full library, then locate the equivalent Quoted Material resource and click Select
This will replace all occurrences of the first resource with the second. You can check that this has occurred in Job Resources under the Estimating subsection of the Analysis menu.
You will now be able to do a comparison for this resource