On the Sumter Electric cooperative (SECO) web page they mention being able to purchase through them a t a discount. Might check it out. It's called "Filter Change Program"http://www.filterchange.coop/?affid=FL016

I used FiltersUSA.com - discount word is Villages. Good prices on all types of filters

I use filters fast. Refrigerator Water Filters, Water Filters, Air Filters, Pool Filters, HVAC Filters, Filters Fast

For all the filers in house

Air filters
 

A 1" filter pleated MERV 7 will work and should be replaced every 3 months. There are two things to consider:
1. The filter rack will need to be adjusted from 4" to a 1" so the filter does not move.
2. The filter is tight fitting so no air bypasses the filter element.

The 4" filter give you more filter surface and is supposed to last longer. For the added expense It is not worth it.
I use pleated filters in my HVAC system and purchase them locally.