MArketpress Product define search

Our setup requires a number of searchable criteria.
1 - Grade
2 - Subject
3 - Type

What would be the most efficient way to set this up for Multisite stores..

These 3 searchables need to remain the same so store owners get to choose from a list only.

At this point Global Tags and Categories are not returning what they should (have this in another post) https://premium.wpmudev.org/forums/topic/global-tags-products-grid-theme-not-functioning

However we need to be able to allow users to search the main site for the 3 fields across the global network.