Under the Bill of Rights you are entitled to representation. You are also entitled to remain silent.
Since the DPD, FBI and the SS claimed that LHO answered questions despite not being represented then he had to waive his right to an attorney, but this form is nowhere to be found.
Can any LNer provide this signed waiver form?
Again, it makes no difference now. Oswald is long dead. Any "rights" that he might have been denied him over 50 years ago are relevant only in the context of a criminal trial that will never take place. Not outside that context. Here on planet earth, we can take all the relevant information into consideration - whether it would have been admissible or not in a trial - and use that to assess whether Oswald was the assassin. Conduct a seance and let Oswald know that you believe his rights were violated if that will make you feel better but resolution of that issue adds or detracts nothing, however, from the case against him.