Sponsor letters are usually for main applicant (parent, spouse) who are applying for a DTV for their dependants (immediate families). Your best bet is to borrow the 500k and have it in your bank account with your name. A lot of people were able to use their bank statement with the money transfered within days of application.

However, if you search this group, someone recently was approved with a sponsor letter from their family. I can't remember which embassy approved this but it's a first I've heard. So maybe try a different country if that's possible for you