For a replacement foam filter for Trigger Systems sumps with the dimensions of 15.5" x 2.5" x 3/4", you should consider a few options:

1. Custom Foam Filter: Many foam filter suppliers allow for custom sizes, so you might find a manufacturer who can create a filter specifically to your stated dimensions.

2. Universal Foam Filters: Look for universal foam filter options that can be cut down to size. Ensure that the material is suitable for filtering water, with the appropriate density and porosity.

Before you purchase, confirm the foam type (e.g., open-cell, closed-cell) and ensure it is compatible with your specific sump system for best performance.