Most jurisdictions accent digitally signed affidavit's. Some require that you first receive a copy of the person's ID and then see them on a recorded video conference in which they 'swear in' (swear that they've been warned to tell the truth and know that there are penalties if they lie.
An alternative and fairly cheap option is to have these people sign in front of local lawyers in their area.
* Disclaimer: each jurisdiction is different, and this field is only now evolving due to covid. You should be able to contact the local Lawyer's Bar association and ask them for the current guidelines.
Good luck