I was able to get mine here^ on 29 Jan, and they are showing as available now (15 Feb early AM). And they're $6.99 instead of $9.99.
All I had to do once I received it was activate the SIM by entering its activation code on tello (and then linking it up with my phone).
As a note, I ordered one from Tello themselves when I set up porting my number/phone from my old carrier and paid for priority mail (SIM card was $9.99 but 100% credited back before I paid the shipping and the account's first month), but usps ended up taking more than a week and still no news, so I ordered online at amazon -- the amazon one got here really fast, so in my opinion it's cheaper, faster, and easier to get it from them on amazon. YMMV.
I don't believe they're available in local stores. Amazon has them for $2-- best if you have Amazon Prime, or have some other stuff you need to order to get free shipping.
https://www.amazon.com/Tello-Mobile-Bring-Your-Phone/dp/B08H5SM9M9
Tello on Amazon sells their new gsm sim for $6.99, a better price than directly from tello.com
https://www.amazon.com/gp/product/B08H5SM9M9/ref=ox_sc_act_title_1?smid=A2LVT5EJVQHP24&psc=1
The sim on Amazon at https://www.amazon.com/gp/product/B08H5SM9M9 has been in and out of stock on and off.
Sometimes it has showed "1 left" etc.
I did manage to get one delivered right away during one of the brief periods when it was in stock.
Right now is shows as available but "Arrives: Sunday, Feb 21" "Fastest delivery: Saturday, Feb 20" which means it is really out of stock at Amazon but they have new stock arriving at the end of this week and you can order from that if you want.
have to be a Tello SIM but not have to buy from tello.com https://www.amazon.com/gp/product/B08H5SM9M9/
To test, you can order a Mint Mobile sim for 99C on amazon with a 7 day trial and see if it works. I'm using a Moto G4 Play as my 2nd line and it's working fine on Tello GSM but I'm pretty sure they said it would not work originally. I ordered the sim for my G7 Plus which qualified and them put it in the G4 to test it and it worked fine. If they won't send you a sim, you can buy from Amazon for $2 Don't port your number get a new one from them for testing purposes and only port your number when you're satisfied.
Even if you need to buy a new phone, you can still keep that phone and use it as a spare WIFI phone at home with Google Voice or give it to someone that would use it. Great as a flashlight, a calculator and for web surfing at home on your WIFI. I think it would still work to call 911 in an emergency. If you have to buy a phone buy a Factory unlocked you could probably get one on Amazon or EBAY for around $100 Check Amazon warehouse deals, the prices drop almost every day and easy return if it does not work. I saw one for $119 earlier today for a G7 Plus 64GB, Factory Unlocked.
You can also buy the Tello sim on Amazon for $2, if you have Prime you'll get it fast in a day or 2. Also be sure to get a referral code from lmloki that gets you a $10 credit that you can use to pay your 2nd bill.
https://www.amazon.com/Tello-Mobile-Bring-Your-Phone/dp/B08H5SM9M9/