We got a new Furnace, and AC unit a few years ago. AC went belly up, and replacement for just it, or both was only a wee bit more for both so we went that way. It has an attached "Air Bear" unit for an easy filter replacement which is a joy to have, versus climbing inside to pry the old one out.

Anyhow, I need to buy new to us 2" thick filters. The tech who services us suggest going here for filters Not saying you have to buy Air Bear type, just they have a huge assortment of sizes, and all different ratings, and for prices I can't buy local and get close. I ended up with what will end up being a MULTI year supply, and did get the free S&H, plus they email me with sale info on a regular basis, the sales are a good thing usually 50% or better off regular prices. Below are some regular prices.

MERV 15's 

MERV 13's

You just need to build to be able to fit standard sized filters in.