In my experience, the seats at those mileage levels do not exist. There is the possibility of some obscure Saturday evening or early Sunday morning flight for a one-way award, but the simple fact is that DL
does not allocate or "give away" many, if at all, award seats at those mileage levels. A rare find, and harder even these days with the capacity reductions DL
has implemented in most markets the past year.
If you should by chance have a AMEX SkyMiles credit card, you can use the "Buy with Miles" option to reduce the cost of a ticket. Otherwise, consider holding on to the miles as they don't expire and find ways to increase accrual, or transfer the miles (a fee applies to do so) to another SkyMiles account to boost their balance with your 25K.
Good Luck with your search, but it can be a very frustrating experience as you've obviously discovered.
watch what you want. you may get it.