Where the money will come from is easy.

 If you look at the timeline, 30 days to sign up, another 30 to wait
untill first payments start rolling in.

Theres less than 60 days left on the original 90 day stakers, so they will
have a sale of new stakers to replace those, and since there still wont
be anything else you can do with your coins people will start buying them up.

This will be the initial buy back fund, the more stakers they sell the more they
will be able to buy back.

Sure everyone will be getting their staked coins back at this time, but that doesnt matter
its too late to enter them in the buyback.