You go to the company that runs the lottery and ask "hi, if I buy 100k tickets, what sort of discount can I get?".
As long as there aren't laws that prevent selling tickets below the nominal price, it's no different than going to a company and asking for a discount when buying bulk quantity of their product.
A discount, in my mind isn’t quite the same as a rebate, even if the net result is the same.
A rebate usually requires the full retail price to be paid up front, with the cost savings deferred to a seperate transaction, often paid by the vendor, when there’s a retail supply chain intervening between purchaser and the supplier.
As long as there aren't laws that prevent selling tickets below the nominal price, it's no different than going to a company and asking for a discount when buying bulk quantity of their product.