What is the biblical support for the practice that only ordained church leaders should administer the only sacraments of baptism and the Lord's Supper?
As St. Paul explains in his letter to the Romans, you are certainly not the first to ask this question ;-)
“Sacraments” are intimately related to “Church”. In Scripture you will find that Jesus himself gives the Holy Spirit to his disciples so that they can give it on. By doing so, He institutes the sacraments and gives authority to the Church to administer them:
This is an example of how the administration is preceded by an ordination:
Note that you could apply the same to Jesus Himself: “As the Father has sent me”.
Another interesting phrase is “if you retain the sins of any, they are retained”: I like to think that Jesus wants these disciples to make sure that everyone gets a chance to receive the gift of forgiveness.
I hope this helps, feel free to contemplate the whole chapter (book) of both excerpts, it often helps to get perspective.
This is easy- it is why none have answered- there is no biblical basis for this practice-(unless one does great theological gymnastics)- we are to be a kingdom of priests unto the heavenly father- we can all fulfill these commanded sacraments wherever we are whenever they should occur.