Yes, you can get it from an ex-professor, if you studied under that prof. You need to submit 3 LORs in CAAPID and they should be generic because they go to all schools. You can get specific school LORs if you wish to send them to schools separately. Caapid accepts scanned letter, but some schools need 'originals'. If you follow caapid instructions and old threads you can get all information from there.
I am just curious, are you applying to US schools or Australia schools? or both?