If it has gone back to the pot, you should be able to buy it back again.
Hopefully no-one else has found it first.
If a plate is on retention already by someone else, it will be unavailable, with no easy way to trace the current owner.
20-years ago we bought a new plate from dvla, and the best way to check availability was via a phone call. A real person might be able to help you out, rather than an online purchase at full price.