So where do I get these? My local post office had no idea what I was even talking about.

Post office is where I always bought them but that was many years ago.  IRC means International Reply Coupon and maybe if you spelled it out for them.  This Page describes them and states that they should be stocked at post offices that have a demand for them.  Maybe print this page and/or try a main post office.

https://pe.usps.com/text/imm/immc3_020.htm

80 Supplemental Services
381 International Reply Coupons
381.1 Description

As of January 27, 2013, the U.S. Postal Service no longer sells international reply coupons. Coupons previously sold by the U.S. Postal Service may no longer be used or exchanged (see 381.2). The following standards apply to international reply coupons:

   The sender of a letter may prepay a reply by purchasing reply coupons that are sold and exchangeable for postage stamps by participating postal administrations in member countries of the Universal Postal Union.
   International reply coupons (in French, Coupons-Reponse Internationaux) are printed in blue ink on paper that has the letters “UPU” in large characters in the watermark. The front of each coupon is printed in French. The reverse side of the coupon, which has text relating to its use, is printed in German, English, Arabic, Chinese, Spanish, or Russian.

Good catch, I didn't see that.  I went back and looked and the page I linked was archived in 2008.  My mistake, my apologies.  Cudos to K9-Bob.

The most secure way to confirm a foreign DX contact is via OQRS. I have confirmed over a hundred contacts using ClubLog and OQRS. It's much a more reliable way to get your contact confirmed than IRC's ever were. Most DXpeditions and many of the more rare DX entities upload their ADIF files to CLubLog and you can use PayPal to get you card. If you aren't using an electronic logbook and LoTW you are missing out.

https://clublog.freshdesk.com/support/solutions/articles/54921-oqrs-online-qsl-requests

I do use electronic but like getting the cards too. I'll check out clublog though.

Using an electronic log and uploading your ADIF to ClubLog guarantees confirmation. You will get a paper card mailed to you along with LoTW confirmation.

Off to sign up.