I’ve had very good experiences of getting SIM cards for my iPhones at the Apple Store. I was a big fan of this, but nowadays, I’m an even bigger fan of staying home, so I personally would not do this today.
Since a few weeks ago, the main Sprint website has been redirecting to the T-Mobile website. For current Sprint customers, the old website is still around, and they are still supporting them. But the new company wants new customers to join the T-Mobile network, and I think that’s why you can’t find the 99 cent Sprint SIM kit on Amazon anymore. (Current Sprint customers already have SIM cards.)
Now, of course, people (like the OP!) will still need to get new Sprint SIM cards for some time — the Sprint network won’t disappear overnight. And there still are Sprint SIM kits available on Amazon, just not for 99 cents.
Right now the lowest price I see is $5.25 (free shipping with Prime or a $25 order.)
https://www.amazon.com/Oberthur-760492036429-Prepaid-SIM-Cards/dp/B07BFJQCJY/
You can even get a Tello SIM kit for $6.99.
https://www.amazon.com/Tello-Mobile-Bring-Your-Phone/dp/B07Z99VXX2
(They are Sprint SIM cards in a Tello package.)
Sprint still sells a SIM kit on their website.
https://www.sprint.com/mysprint/shop/accessory/ao_details.jsp?accSKU=121700244&accCatId=78300001
It’s $10, though. They used to regularly sell them for $3, but I don’t know if they’re going to do that anymore.
I don’t know if this helps the OP (who already found an Ebay listing for $4) but perhaps it can help someone.