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)

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