Can I amend my 2019 filing with TurboTax if I didn't use TurboTax to file originally? Where do I purchase the software for amendment? Thx!

It is possible to amend a tax return prepared elsewhere using TurboTax but it will be a real pain.  It would be much easier to use the software used originally to prepare the returns than to use TurboTax.   

In order to use TT software, first you have to purchase the desktop software for that year and download it to a full PC or Mac.  Then you have to re-create the tax return EXACTLY as it looked when sent to the IRS.  You then tell the software you will file by mail in order to "trick" the software into thinking you filed.  THEN, you use that same software to prepare a Form 1040X in order to amend the return, and print, sign and mail the Form 1040X to the IRS.  It takes the IRS about 3-4 months to process an amended return.  It is too late to receive a refund for an amended return for any year earlier than 2017.

To file a return for a prior tax year

 If you need to prepare a return for 2017, 2018, or 2019 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return.

https://ttlc.intuit.com/replies/7441660

CAN I USE TURBOTAX TO AMEND IF THE RETURN WAS PREPARED SOMEWHERE ELSE?

If you did not use TurboTax to prepare your tax return, it is possible to amend the return using TurboTax but it will really be a pain.  It would be better to use the software or service used to file originally.  If a paid preparer made a mistake on your tax return, that tax preparer should do the amended return for you at no charge.

In order to use TurboTax to amend your return, you need to purchase the desktop software for the tax year and re-create the return EXACTLY as it appeared when it was filed with the IRS/states.  Every line has to be the same.  

Then you have to "trick" the software into thinking you filed by saying you will mail the return. (but do not mail it)   AFTER that, you use the software to prepare a Form 1040X, which has to be printed, signed and mailed to the IRS.  It takes the IRS about 4-6 months to process an amended return

 If you need to prepare a return for 2017, 2018, or 2019 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.