Where can I buy MRE's, meals ready to eat?

Military surplus stores usually have them.

I second @funkdaddy‘s suggestion. I have never seen them in chain stores. Good luck, and be sure to change them out every couple years.

Ebay,

The sell them at gun shows. Check for the next show in your area and stock up.
By the way, don’t be surprised if you are the only female in the building.

There are lots of e-stores that sell them and now there are ones that specialize in emergency prepping with freeze dried items that are often tastier and overall cheaper than MRE’s. Honeyville is a place that sells freeze dried that a lot of preppers I know speak highly of.

Go to a local REI store get a Membership card and get 10 % of your purchases back ( next year as a coupon for the store ). Real MRE’s real good equipment and supplies.

Sportsman’s Guide seems to have warehouses of them because every flyer I get from them advertises them on sale.

You can also get some in camping stores like REI or Bass Pro, or in the camping section of stores like Target and Walmart. You can also find them on Amazon