Even if you don't buy the ARRL Antenna course, you can buy the $39.95 "ARRL
Antenna Book" which comes with a customized version of EZNec with models of
every antenna discussed in the book.
I started with EZNEC using the customized (and somewhat crippled) ARRL
version, but the program was essentially free with the ARRL book. When I
decided that I wanted to model my own designs and save the modifications
that I made to existing ARRL files, I bought the full program (which can
still open and manipulate all the ARRL models).