The endorsement has to be signed by an FAA certified flight instructor under part 61.31
There are a few exemptions though:
If you have tailwheel experience prior to the 1991 cut-off then no endorsement is required.
If you should happen to have a tail-skid rather than tailwheel, you are also technically exempt.
I believe the FAA also occasionally grant authorisations if you have significant tailwheel experience under another licence authority. Contact a FSDO for advice on this.
Otherwise you need to find a CFI who is also tailwheel authorised. Maybe you could combine it with a BFR.