@ijbaby , while @DoninGA has adequately answered your actual question on how to get a copy of the original return, there are few more things you need to note .
(a) Because you are not amending but changing one original filing to another original filing -- 1040 to 1040-NR, and because lack of support of the latter from TurboTax, you will have to file by mail.
(b) Prepare the form 1040-NR just as if it is an original filing, print sign and date; Write on top Corrected Return
(c) you can prepare a 1040-X and fill in the requisite numbers as far as possible ( note line numbers will not match but categories will. For reason for amend declare that this was and "unintended mistake of using the wrong form"
(d) Take a printed copy of the original filed return, sign and date and write on top Original filing Return.
(e) put the whole package together and mail in trackable mail. ( You just want proof of delivery but do not ask for signature -- that may delay delivery/acceptance )
There after , wait for the IRS to come back.
Does this help ? Is there more one of us can do for you ?
pk
