I read how Steven Spielberg named his production company "Amblin"
after an early film he did, which apparently got him his break into
directing.

Being a fan of him and his films, I would like to know where I could
download a copy of this film.

Any other of his earlier films would also be welcome.

Good luck.  Amblin (Spielberg's 26-minute short film from 1968) has
not been publically available since Jaws first came out.  Spielberg
has decided to keep it private.  So it's hard enough viewing, let
alone downloading.  However, you can view a few short clips from it in
one of the extras on the Jaws DVD (perhaps you've already seen this
footage).