I went ahead and looked assuming you are in the US. You are right, there really is a shortage of them right now. Maybe it is COVID shortages and whatnot? Even the Sea Monkey's own site (here) seems to be low on options.
Normally amazon has a bunch of stuff. They have a couple kits, but they are pretty expensive compared to normal. They do have the basic packets if you want to find your own container (here). If you have a Joann fabric store near you, they might have it (mine has them in stock, link here). In theory, Hobby Lobby might have them in store, but their website does not allow you to see if it is carried in store or not. The last option I have for you is Ebay. I have bought some from Ebay in the past as they did just fine. Just make sure it is a reputable seller.
You can also try aqua dragons, which are pretty much the same thing (there are some differences, and many prefer Sea Monkey brand, but it works of you can not find anything else). Amazon has them for $19.99 (here).
And today it is back to 11.99
And today, the refill kit is 6.99